Aviat ion Safety Counci l Ta ipe i , Ta iwan

GE791 Accident Investigation Factual Data Collection

Group Report

Air Traffic Service Group

October 28, 2003

ASC-FRP-03-10-001

Intentionally Left Blank

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I. Team Organization

Chairman:

Walter Chang / Senior Engineer, ASC, ROC

Members:

1. Michael Guan / Deputy Director of Investigation Lab, ASC

2. Ming-Hao Yang / Assistant Engineer, ASC

3. Chin Wan Lee / Chief of TAMC, CAA

4. Wei-chien Sun / ATR 72 Pilot, TNA, ROC

5. Claude Azibane / BEA

6. Edoardo d’Aniello / Flight Safety Officer, ATR Aircraft Manufacture Corporation

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II. History of Activities

Date Description

91/12/21

1. The CAA, MOTC provided related documents for investigation of ATS group.

2. Obtained ATC recordings and transcripts from Taipei Area Control Center.

91/12/23 Obtained weather radar data and other weather information from Central Weather Bureau.

91/12/26 Obtained Airport Surveillance Radar data

91/12/31 Obtained Weather information from aircrafts near the accident site.

92/01/15 Interviewed one dispatcher of Trans Asia Airways.

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III. Factual Description

1.7 Meteorological Information

1.7.1 Weather Synopsis

The surface weather and upper air conditions for eastern Asia were summarized from the Central Weather Bureau (CWB) Weather Depiction Charts at 1200 and 1800 UTC. The charts revealed a low pressure center moving easterly in the sea area near Kyushu Island, Japan. A stationary front extended in a southwesterly direction from the low pressure center to central Taiwan. Broken to overcast cloud with rain or temporary light rain were occurring to Taiwan. The surface temperature of plus 20℃ was being reported near the site of the accident.

The CWB’s 850 hPa analysis charts (recorded about 5,000 feet MSL) at 1200 UTC on Dec. 20 and 0000 UTC on Dec. 21 indicated an area of low pressure with the center located in the sea area near Kyushu Island, extended in a southwesterly direction to southern China. A trough of temperature located near 110° E. The temperature were plus 11℃ to plus 13℃ in the area of Taiwan Strait with moisture evident from southern China to Taiwan and Ryukyu Islands.

The CWB’s 700 hPa analysis charts (recorded about 10,000 feet MSL) at 1200 UTC on Dec. 20 and 0000 UTC on Dec. 21 indicated a trough of low pressure located in the western China. A southwesterly flow located over central and southern China with gusty winds in the coast area of southern China. The temperature were plus 2℃ to plus 4℃ in the area of Taiwan Strait with moisture evident from southern China to Taiwan and Ryukyu Islands.

The CWB’s 500 hPa analysis charts (recorded about 18,000 feet MSL) at 1200 UTC on Dec. 20 and 0000 UTC on Dec. 21 indicated a trough of low pressure located in the western China. A strong southwesterly flow located over southern China. The temperature were minus 9 ℃ to minus 10 ℃ with moisture evident in the area of Taiwan Strait.

The CWB’s 400, 300 and 200 hPa analysis charts (recorded about 24,000, 30,000 and 40,000 feet MSL respectively) at 1200 UTC on Dec. 20 and 0000 UTC on Dec. 21 indicated a jet stream located in the southern China. The

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temperature were minus 21℃, minus 37℃ and minus 55℃ respectively with moisture evident decreased with time in the area of Taiwan Strait.

Lightnings were detected in the sea area east and northeast of Taiwan and there was no lightning reported in Taiwan and Taiwan Strait from 1720 to 1820 UTC.

The Upper level wind and temperature data at the way points “CHALI”, “CANDY”, “SIKOU” and “MAKUNG” calculated from the MM5 Mesoscale Model of Taipei Aeronautical Meteorological Center (TAMC) is included appendix 1-1. Total Air Temperatures (TAT) and derived Static Air Temperatures (SAT) from FDR are as followers:

-15

-10

-5

0

5

10

15

20

17:0

3:32

17:0

5:43

17:0

7:24

17:0

9:05

17:1

0:47

17:1

2:28

17:1

4:10

17:1

5:52

17:1

7:33

17:1

9:15

17:2

0:57

17:2

2:39

17:2

4:21

17:2

6:02

17:2

7:44

17:2

9:39

17:3

1:32

17:3

3:26

17:3

5:20

17:3

7:13

17:3

9:07

17:4

1:05

17:4

2:59

17:4

4:52

17:5

0:21

17:5

2:14

Time (UTC)

°C

TAT (℃)

SAT (℃)

Figure1.7-1 TAT and SAT along the track of FDR.

The GMS-5 infrared imager data at 1624, 1731 (Appendix 1-2) and 1831 UTC and the Doppler weather radar data indicated some convective movement developed from the coast area of southern China and moved to Taiwan with the flow. Convective clouds were found in Eastern Chinese Sea, central and northern Taiwan and Taiwan Strait. From departure to way point “CHALI” along the track of GE791, tops of the highest cloud layer were mainly about 35,000 feet MSL with temperature about minus 49℃. The tops became lower to 24,000-26,000 feet MSL with temperature about minus19-minus 25℃ from way point “CHALI” to “SIKOU”. It became further lower to about 20,000 feet MSL with some gaps of the cloud layers from way point “SIKOU” to “MAKUNG”. From way point “MAKUNG” to the accident site, it became higher to about

3533 3563

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29,000 feet.

There was no any AIREP received around the time of the accident.

1.7.2 Surface Weather Observations

Surface weather observations surrounding the accident site and takeoff airport were as follows:

CKS International Airport (RCTP) [located 253 kilometers northeast of the accident site]: Time— 1700 UTC; Wind— 040 degrees at 11 knots; Visibility— greater than 10 kilometers; Clouds— scattered 800 feet, broken 1200 feet, overcast 4000 feet; Temperature— 20 degrees Celsius; Dew Point—20 degrees Celsius; QNH—1014 hPa; Trend Forecast-TEMPO Visibility-3000 meters; Present Weather-moderate rain; Clouds-broken 800 feet overcast 3000 feet=

Time— 1800 UTC, Wind— 040 degrees at 8 knots; Visibility— 7000 meters; Present Weather— light rain; Clouds— scattered 800 feet, broken 1200 feet, overcast 4000 feet; Temperature— 19 degrees Celsius; Dew Point— 19 degrees Celsius; QNH— 1014 hPa; Trend Forecast-TEMPO Visibility-3000 meters; Present Weather-moderate rain; Clouds-broken 800 feet overcast 3000 feet; Remark rain amount 0.50 millimeters =

Makung Airport (RCQC) [located 21 kilometers northeast of the accident site]: Time— 1700 UTC, Wind— 020 degrees at 16 knots gusting 28 knots; Visibility— 6000 meters; Present Weather— light rain; Clouds— scattered 600 feet, broken 1000 feet, overcast 4000 feet; Temperature— 20 degrees Celsius; Dew Point— 19 degrees Celsius; QNH— 1013 hPa; Trend Forecast-no significant change; Remark rain amount 1.30 millimeters =

Time— 1800 UTC, Wind— 040 degrees at 15 knots gusting 27 knots; Visibility— 7000 meters; Present Weather— light rain; Clouds— scattered 600 feet, broken 1000 feet, overcast 4000 feet; Temperature— 20 degrees Celsius; Dew Point— 19 degrees Celsius; QNH— 1012 hPa; Trend Forecast- no significant change; Remark rain amount 0.30 millimeters =

Kaohsiung International Airport (RCKH) [located 137 kilometers southeast of the accident site]: Time— 1700 UTC, Wind— 360 degrees at 5 knots; Visibility— 6000 meters; Present Weather— light rain; Clouds— scattered 800 feet, broken 1500 feet, overcast 4500 feet; Temperature— 20 degrees Celsius; Dew Point— 19 degrees Celsius; QNH— 1012 hPa;

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Trend Forecast-no significant change; Remark rain amount 0.75 millimeters =

Time— 1800 UTC, Wind— 340 degrees at 6 knots; Visibility— 6000 meters; Present Weather— light rain; Clouds— scattered 800 feet, broken 1500 feet, overcast 4500 feet; Temperature— 20 degrees Celsius; Dew Point— 19 degrees Celsius; QNH— 1011 hPa; Trend Forecast- no significant change; Remark rain amount 0.50 millimeters =

Chiayi Airport (RCKU) [located 96 kilometers east of the accident site]: Time— 1800 UTC, Wind— 090 degrees at 6 knots; Visibility— 3200 meters; Present Weather— light rain and mist; Clouds— scattered 1000 feet, broken 2500 feet, overcast 5000 feet; Temperature— 19 degrees Celsius; Dew Point— 19 degrees Celsius; QNH— 1013 hPa; Trend Forecast-no significant change; Remark rain 1.8 millimeters =

Chinmen Airport (RCBS) [located 151 kilometers northwest of the accident site]: Time—1800 UTC, Wind—030 degrees at 4 knots; Visibility—4500 meters; Present Weather—light rain; Clouds—few 800 feet broken 2200 feet broken 5000 feet; Temperature—17 degrees Celsius; Dew Point—15 degrees Celsius; QNH—1016 hPa; Trend Forecast-no significant change; Remark rain amount 2.00 millimeters =

Rain amount records of CWB surrounding the accident site were as follows:

TIME (UTC) STATION [location from the accident

site] 15~16 16~17 17~18 18~19 19~20

TAICHUNG [146 km northeast]

0 0 1.1(mm) 0.3(mm) 0.6(mm)

CHIAYI [100 km east]

T 0.5(mm) 2(mm) T 2(mm)

TAINAN [94 km southeast]

0.5(mm) 0.5(mm) 0.5(mm) 0 1(mm)

PENGHU [15 km northeast]

T 0.8(mm) 0 0.5(mm) 2.5(mm)

TUNGCHITAO [33 km southeast]

T 1(mm) 0 1(mm) 1.5(mm)

T: trace

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1.7.3 Weather Advisories

The Taipei Aeronautical Meteorological Center (TAMC) had responsibility for issuing Significant Meteorological Information (SIGMETs) for the Taipei Flight Information Region (FIR) and Significant Weather Prognostic Charts (SIGWX Charts). The following SIGMETs were valid before and after the time of the accident:

RCTP SIGMET 2 VALID 200600/201000 RCTP- TAIPEI FIR EMBD TS OBS AND FCST S OF N27 CB TOP FL 450 MOV ENE 10 KT NC=

[SIGMET 2 Valid at 200600 UTC to 201000 UTC for Taipei FIR; Embedded thunderstorm observed and forecasted south of N27; Cumulonimbus top— FL 450; Moving east-northeasterly at 10 knots; Intensity— no change.]

RCTP SIGMET 3 VALID 202030/210030 RCTP- TAIPEI FIR EMBD TS OBS AND FCST N OF N23 AND E OF E118 CB TOP FL 400 MOV ENE 10 KT WKN=

[SIGMET 3 Valid at 202030 UTC to 210030 UTC for Taipei FIR; Embedded thunderstorm observed and forecasted north of N23 and east of E118; Cumulonimbus top— FL 400; Moving east-northeasterly at 10 knots; Intensity— weaken.]

According to the SIGWX charts issued from TAMC for SFC to FL100 and FL100 to FL250, valid at 1800 UTC on Dec. 20 and 0000 UTC on Dec. 21 (SIGWX charts for FL100 to FL250 are in Appendix 1-3), the forecasted weathers of Taipei to Penghu Islands were as follows:

Precipitation in the form of rain with broken to overcast cloud. Cloud ceilings were 1,500 to 3,000 feet and cloud tops were equal to or greater than 25,000 feet. Stratus (St) and stratocumulus (Sc) overlaid by altostratus (As) and altocumulus (Ac). Isotherm of 0℃ was at about FL120. No icing or turbulence indicated. Wind and temperature aloft: FL100, Wind— 230 to 250 degrees at 15 to 30 knots; Temperature— plus 2 to plus 5 degrees Celsius; FL180, Wind—240 to 250 degrees at 40 to 50 knots; Temperature— minus 9 to minus 13 degrees Celsius.

The following SIGMETs were issued from Hong Kong Observatory (HKO) and valid in Hong Kong Control Area (CTA) around the time of the accident:

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VHHK SIGMET 4 VALID 201340/201740 VHHH- HONG KONG CTA EMBD TS FCST IN AREA W OF E114 BTN N18 AND N20 CB TOP FL350 MOV NE 20 KT NC=

[SIGMET 4 Valid at 201340 UTC to 201740 UTC for HONG KONG CTA; Embedded thunderstorm forecasted in area west of E114 and between N18 and N20; Cumulonimbus top— FL 350; Moving northeasterly at 20 knots; Intensity— no change.]

VHHK SIGMET 5 VALID 201635/202035 VHHH- HONG KONG CTA EMBD TS FCST IN AREA (1) N OF N21 E OF E115 CB TOP FL350 MOV E 15 KT WKN AND IN AREA (2) E OF E113 BTN N18 AND N20 CB TOP FL400 MOV E 20 KT INTSF=

[SIGMET 5 Valid at 201635 UTC to 202035 UTC for HONG KONG CTA; Embedded thunderstorm forecasted in area (1) north of N21 and east of E115; Cumulonimbus top— FL 350; Moving easterly at 15 knots; Intensity—weaken. (2) east of E113 and between N18 and N20; Cumulonimbus top— FL 400; Moving easterly at 20 knots; Intensity— intensify.]

VHHK SIGMET 6 VALID 202035/210035 VHHH- HONG KONG CTA EMBD TS FCST E OF E115 BTN N18 AND N20 CB TOP FL400 MOV E 20 KT NC=

[SIGMET 6 Valid at 202035 UTC to 210035 UTC for HONG KONG CTA; Embedded thunderstorm forecasted in area east of E115 and between N18 and N20; Cumulonimbus top— FL 400; Moving easterly at 20 knots; Intensity— no change.]

According to the SIGWX chart issued from HKO for FL100-FL250 of eastern Asia, valid at 1800 UTC on Dec. 20 (Appendix 1-4), the forecasted weathers of Taipei to Penghu Islands were as follows:

Isotherm of 0℃ was at about FL120. Moderate icing was at FL120 and higher. Moderate turbulence was at FL220 and lower.

The following SIGMETs were issued from Tokyo Aviation Weather Service Center (TAWSC) and valid in Naha FIR around the time of the accident:

RORG SIGMET 3 VALID 201220/201620 RJAA- NAHA FIR FRQ TS FCST IN AREA BOUNDED BY N27E126 N27E127 N29E130 N30E130 N30E127 N29E126 AND N27E126 MOV NE 20 KT

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NC=

[SIGMET 3 Valid at 201220 UTC to 201620 UTC for Naha FIR; Frequent thunderstorm forecasted in area bounded by N27E126 N27E127 N29E130 N30E130 N30E127 N29E126 AND N27E126; Moving northeasterly at 20 knots; Intensity— no change.]

RORG SIGMET 4 VALID 202110/210110 RJAA- NAHA FIR MOD TO SEV TURB FCST IN AREA BOUNDED BY N24E124 N24E127 N27E130 N30E130 N30E127 N28E126 N26E124 AND N24E124 FL350/390 MOV ENE 20 KT INTSF=

[SIGMET 4 Valid at 202110 UTC to 210110 UTC for Naha FIR; Moderate to severe turbulence forecasted in area bounded by N24E124 N24E127 N27E130 N30E130 N30E127 N28E126 N26E124 AND N24E124 at FL350/390; Moving east-northeasterly at 20 knots; Intensity— intensify.]

According to the SIGWX charts issued from TAWSC for surface to 14,000 meters height (Appendix 1-5), southwest Taiwan and Penghu area were not included. The forecasted weathers of Taipei to Penghu Islands were as follows:

For SIGWX chart valid at 1800 UTC on Dec. 20, moderate icing was at FL120 to FL240 and moderate turbulence was at FL20 to FL380 in central and north Taiwan and the sea area of northeast Taiwan.

For SIGWX chart valid at 0000 UTC on Dec. 21, moderate icing was at FL80 to FL220 and moderate turbulence was at FL20 to FL320 in east Taiwan and it’s sea area.

1.7.4 Weather Information Provided To the Pilots

From the interview with the dispatcher for the accident flight, the flight release contained Meteorological Reports (METARs) and Terminal Aerodrome Forecasts (TAFs) of RCTP, VMMC and VHHH at 1000 UTC, infrared satellite image at 1000 UTC and wind/temperature forecast of FL020, FL050, FL100, FL150 and FL200 eastern Asia at 1700 UTC.

The following TAFs were issued at 1000 UTC and valid from 1200 UTC on Dec. 20 and 1200 UTC on Dec. 21:

CKS International Airport (RCTP): Wind— 030 degrees at 8 knots;

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Visibility— 8000 meters; Clouds— few 1000 feet, broken 2000 feet, broken 5000 feet. Temporary at 201200 UTC to 202400 UTC, Visibility— 4000 meters; Present Weather— moderate shower rain; Clouds— scattered 500 feet, broken 1200 feet, overcast 3000 feet. Temporary at 210000 UTC to 211200 UTC, Wind— 030 degrees at 15 knots; Visibility— greater than 10 kilometers.

Macau International Airport (VMMC): Wind— 030 degrees at 10 knots; Visibility— 8000 meters; Clouds— few 1000 feet, scattered 1800 feet, broken 6000 feet. Temporary at 201200 UTC to 202000 UTC, Wind— 090 degrees at 10 knots; Visibility— 4000 meters; Present Weather— moderate rain. Temporary at 202000 UTC to 210200 UTC, Visibility— 3000 meters; Present Weather— light rain and mist.

Hong Kong International Airport (VHHH): Wind— 030 degrees at 12 knots; Visibility— 9000 meters; Clouds— few 1200 feet, scattered 2500 feet, broken 8000 feet. Temporary at 201200 UTC to 201500 UTC, Wind— 120 degrees at 8 knots. Temporary at 201200 UTC to 201800 UTC and 202100 UTC to 210300 UTC, Visibility— 4000 meters; Present Weather— moderate rain and mist.

The flight information station in CKS International Airport provided TAFs of Southeast Asia valid from 1200 UTC, GMS-5 infrared satellite image at 1330 UTC, ICAO Area G SIGWX Chart of FL 250-630 valid until 1800 UTC, wind/temperature forecast of FL180 for Asia/Europe and FL300, FL340 and FL390 for East Asia valid until 0000 UTC on Dec. 21. The SIGWX chart was issued from London World Area Forecast Center and the upper level wind and temperature forecast were issued from Washington World Area Forecast Center.

1.7.5 Doppler Weather Radar Information

Weather radar data were collected from the WSR-88D Doppler weather radar sites located in Mt. Wufan, Taipei County (RCWF, located 295 kilometers northeast of the accident site and 55 kilometers east of RCTP), and the METEOR 1500S Doppler weather radar sites located in Chiku, Tainan County (RCCG, located 74 kilometers southeast of the accident site and 244 kilometers south-southwest of RCTP). The radars are operated by the CWB.

Weather radar images from RCWF and RCCG for 1700 to 1800UTC, at the

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elevation angles of 0.5, 1.4(1.45), 2.4, 3.4 and 4.3 degrees were reviewed. The heights of the radar beam center in the waypoints along the GE791 track are as follows:

Elevation Angles

0.5° 1.4 1.45° 2.4° 3.4° 4.3°

Beam width

RCWF

CHALI 9700 ft 17000 ft 24200 ft 7200 ft

CANDY 12500 ft 21500 ft 30500 ft 9000 ft

SIKOU 17200 ft 28700 ft 40200 ft 11500 ft

MAKUNG 24300 ft 38900 ft 53500 ft 14600 ft

RCCG

CHALI 11500 ft 20800 ft 31000 ft 41300 ft 50500 ft 9800 ft

CANDY 8400 ft 15900 ft 24200 ft 32600 ft 40100 ft 7900 ft

SIKOU 5000 ft 10200 ft 15900 ft 21700 ft 26900 ft 5500 ft

MAKUNG 2900 ft 6400 ft 10200 ft 14000 ft 17500 ft 3600 ft

The computed echo intensities along the GE791 track are as follows:

Time

（UTC） Aircraft Altitude （ft）

Echo Intensity

(dBZ)

Note

1713 10200 17.7

1714 11000 18.3

1715 11900 18.8

1716 12700 25.3

1717 13400 22.1

1718 14200 14.3

1719 14900 13.3

1720 15600 14.7

1721 16200 17.0

1722 16700 20.3

1723 17300 20.5

1724 17700 20.2

1725 18000 19.9 Waypoint “CHALI”

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1726 18000 19.2

1727 18000 18.9

1728 18000 19.6

1729 18000 19.4

1730 18000 18.8

1731 18000 18.5 Waypoint “CANDY”

1732 18000 15.8 CVR: Looks like it’s iced up…. look at my side your side is also iced up right

1733 18000 22.0 CVR: There’s not enough moisture outside minus twelve degrees

1734 18000 15.3 CVR: Oh it’s icing up FDR：Airframe De-Icing on

1735 18000 15.0

1736 18000 10.1

1737 18000 11.8 FDR：Airframe De-Icing off

1738 18000 10.5 Waypoint “SIKOU”

1739 18000 7.4

1740 18000 3.7

1741 18000 4.5 FDR：Airframe De-Icing on 1742 18000 <MDS1 1743 18000 <MDS

1744 18000 <MDS CVR: It’s iced up quite a huge chunk

1745 18000 <MDS

1746 18000 <MDS

1747 18000 3.8

1748 18000 8.9 Waypoint “MAKUNG”

1749 18000 2.3

1750 18000 2.0 CVR: Wow it’s a huge chunk CVR: What an ice

1751 18000 <MDS CVR: Just as long as no more moisture because we have moisture now

CVR: So do you want to move up or ah severe icing up

1752 18000 <MDS CVR: It’s severe icing up

The Plan Position Indicator (PPI) of radar images with the ground track of GE791 superimposed are contained in Appendix 1-6. The cross section charts of radar images with the track of GE791 superimposed are contained in Appendix 1-7. 1 The smallest incoming signal that will be detected, and produce a discernable target, is

referred to as the minimum discernable signal (MDS)

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Weather radar data indicated an area of higher echo intensity about 25-45dBz, moving east-northeasterly with the clouds in the northern part of Taiwan Strait. The length of about 200 kilometers and width about 100 kilometers and located from FL60 to FL120. Tops of the highest cloud layer overlaid the area were about 35,000 feet MSL. The GE791 flew above the area from before waypoint “CHALI” to waypoint “CANDY”.

1.7.6 Weather information from aircrafts near the accident site

The ASC collected the flight data of the aircrafts around the accident site to get the better understandings of the weather conditions. According to the TACC radar recordings, two aircraft with the assigned beacon codes of 3533 and 3563 flew over the accident site. After the flight data were synchronized, both flight tracks were superposed with the track of GE791. Figure 1.7-2 displays the results. The blocked area in figure 1.7-2 is the area where GE791 disengaged the airframe De-Icing device until it disappeared from the radarscope. The flight track of the beacon code 3563 aircraft was similar to the GE791’s, also via airway A1 to over fly the accident site from 1741:24 UTC to 1745:26 UTC. As the beacon code 3563 aircraft descent from FL350 to FL240, the average wind was 260 degrees at 88 knots and the Total air temperature (TAT) increased from minus 16.5℃ to plus 3.3℃. Detail winds and TAT are shown in Figure 1.7-3.

The beacon code 3533 aircraft (A300-600R) over flew the accident site from 1807:55 UTC to 1815:18 UTC. The CI611 flight track labeled as “c” was on the right hand side about 22 km of airway A1 and “d” in Figure 1.7-2. As the beacon code 3533 aircraft descent from FL300 to FL240, the average wind was 260 degrees at 66 knots and the TAT increased from minus 10.5℃ to plus 2.5℃. Detail winds and TAT are shown in Figure 1.7-4. The witness statements of the pilots are address in section 1.18.3.10.

The variation of winds and TAT with altitude information of both aircrafts is shown in Figure1.7-5.

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Figure1.7-2 Superposition of the flight tracks of beacon code 3533 and 3563 aircrafts near the accident site of GE791.

3533 3563

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Figure1.7-3 The wind condition and TAT of the beacon code 3563 aircraft（The green area was flown pass by the accident site.）

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Figure1.7-4 The wind condition and TAT of the beacon code 3563 aircraft （The blue area was flown pass by the accident site.）

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Figure 1.7-5 The variation of wind condition and TAT with altitude of the beacon code 3533 and 3563 aircrafts.

3533-TAT

3563-Wind Speed

3533-Wind Speed

3533-Wind Speed 3533-Wind Direction

3563-Wind Speed 3563-Wind Direction

3533-Wind Direction

3563-Wind Direction

3533-TAT

3563-TAT

3563-TAT

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1.8 Aids to Navigation

There were no known malfunctions with the aids to navigation involved in this accident.

1.9 Communications

There were no known difficulties with internal or external communications.

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1.16.4 Liquid water content and icing

1.16.4.1 General information on super-cooled water drops/droplets and liquid water content (LWC)

The following cloud and precipitation water droplets and drops size scale is usually used by the international community:

- Small droplets: < 10 µm (stable clouds), - Medium droplets: 10 to 40 µm (stable clouds), - Large droplets: > 40 to 100 µm (unstable clouds), - Drizzle drops: 50 to < 500 µm, - Rain drops: ≥ 500 µm.

Wavelengths of WSR-88D and METEOR 1500S are around 10 cm. With this kind of radar, reflection from drops or "wet" crystals with a size of 100 µm or more is optimum. With droplets size of 50 to 80 µm (large droplets of clouds or small droplets of drizzle), detection is also reliable whether droplets are particularly numerous and mixed with larger ones. The weather radar imagery allowed pointing out that among detected droplets, diameters of at least 40 to 100 µm could represent a large part of the considered LWC, as well as larger super-cooled water drops coming from drizzle or rain precipitation.

The methods for liquid water content (LWC) estimation are as follows:

The first is the "Penn State University" diagram and formula (appendix 1-8) relate the number of drops per cubic centimeter and the radar reflectivity factor for various droplet radii. Given the number of drops per cubic centimeter and the diameter of the drops, a liquid water content (LWC) can be calculated by the formula:

LWC (g/m3) = π/6*N (cm

3)*D3

(µm) / 106,

It’s rather valid for a monodisperse droplet distribution (drops or droplets of same size, hypothesis in stable conditions) the following theoretical results of various droplets number per cm3 (N) -versus radar reflectivity factors (R) and various droplets radii (0.5 D)- and liquid water content are obtained:

- R = 00 dBz, D = 60 µm: N = 20 (cm-3) and LWC = 2.26 g/m3, - R = 00 dBz, D = 80 µm: N = 2 (cm-3) and LWC = 0.54 g/m3, - R = 05 dBz, D = 80 µm: N = 8 (cm-3) and LWC = 2.14 g/m3,

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- R = 05 dBz, D = 100 µm: N = 3 (cm-3) and LWC = 1.60 g/m3, - R = 10 dBz, D = 200 µm: N = 0.15 (cm-3) and LWC = 0.63 g/m3, - R = 15 dBz, D = 500 µm: N = 0.003 (cm-3) and LWC = 0.20 g/m3, - R = 20 dBz, D = 500 µm: N = 0.005 (cm-3) and LWC = 0.33 g/m3, - R = 20 dBz, D = 1000 µm: N = 0.0001 (cm-3) and LWC = 0.05 g/m3, - R = 30 dBz, D = 1000 µm: N = 0.001 (cm-3) and LWC = 0.52 g/m3, - R = 40 dBz, D = 2000 µm: N = 0.0002 (cm-3) and LWC = 0.84 g/m3.

Another formula giving results good enough is drawn from the "Forecaster's Guide On Aircraft Icing", US Air Weather Service (March 1980):

LWC (g/m3) = 0.348 (W0 - W1) (g/kg) P (hPa) / T (°K).

Where W0 and W1 are respectively the saturation mixing ratio at cloud base and at flight level, P and T pressure and temperature at flight level. Results obtained from that formula are considered as the upper limit of LWC in prevailing cumuliform clouds at flight level. In stratiform clouds those LWC computed values are about the half.

At last, a relationship drawn from the "Monthly Weather Review" (July 1972) about "Vertically Integrated Liquid Water", which directly relates liquid water content (g/m3) to radar reflectivity (dBz) by the formula:

LWC (g/m3) = 3.44*10-3Z4/7, where Z=10dBz/10

1.16.4.2 Icing information

1.16.4.2.1 Types of ice accretion on exposed parts of the airframe

For all the flight in cruise of GE791, SAT was about minus 10 °C to minus 12 °C and TAT was less than or equal to 0 °C, it can be assumed that:

- Small super-cooled droplets at a temperature less than minus 10 °C immediately froze at the impact,

- Larger drops, after spreading out on the impact zone, froze as the super-cooling process instantaneously ends in such air temperature conditions as low as minus 11/ minus 12 °C.

The following are the FAA AIM definitions of the two different type of ice:

- Rime ice formed by small super-cooled water droplets of stable

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clouds, which instantaneously freeze at the impact: "rough, milky and opaque ice",

- Clear ice formed by larger super-cooled water droplets or drops of unstable clouds or precipitation, which, more or less, spread out on the airframe and on or under the airfoil (depending on the angle of attack) prior to freeze: "glossy, clear or translucent ice".

1.16.4.2.2 Calculations of ice accretion on the airframe

1.16.4.2.2.1 General

Passing through clouds including super-cooled water drops, cloud drops or liquid precipitation, an aircraft accretes more or less ice. When these drops hit the airframe the super-cooling status ceases, instantaneously or gradually after spreading out, depending on heat transfer. Resulting ice accretes on impingement parts (wing leading edge, tailplane leading edge, nose, antennas...). Type, shape, thickness of ice deposit depends on:

- Atmospheric conditions: ambient temperature, liquid water content, drops mean effective diameter, horizontal and vertical extent of cloud layers;

- Aircraft conditions: true airspeed, skin temperature, exposure duration, angle of attack, level of icing protection systems, aircraft geometrical characteristics.

The main parameters to consider are directly relative to the airframe ability to accrete ice, they are the "droplets trajectory parameters", as Figure1.16-1:

- The "total collection efficiency": ρ = ∆e / h, - The "local impingement efficiency": β = δe / δS

where ∆e is the free-stream collection (impingement) width, h is the projected frontal height of the airfoil, δe is the distance between two trajectories apart in the free-stream and δS is the curve distance between the impingement points on the airfoil of these two trajectories

Both parameters ρ and β depend on the angle of attack α. Thus at very low angle of attack tending to 0°, the "total collection efficiency" ρ is near to 1 and it is decreasing at the same time α is increasing.

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Figure1.16-1 Droplets trajectory parameters

1.16.4.2.2.2 Apply to ATR aircraft

The amount and type of ice accretion depending on the liquid water content of the layer in which the aircraft is flying, the evaluation of this parameter is essential. Different tests were made, for instance using the hot wire liquid water content meter Johnson-Williams or the "témoin CEV2".

In this report the reference is the "témoin CEV" which consists of a little cylindrical bar of one inch diameter (2.54 cm). Mounted on flight tests aircraft, it uses wind-tunnel results between the LWC and the ice caption speed. That ρ value is always close to 1 because it does not depend on the angle of attack since the bar is a cylinder. Although the estimations are valid for a hypothesis of a water drop MED of about 20 µm, they may considered as representative of certain icing conditions and the use of the "Lucas Aerospace" diagram (Appendix 1-9) at different "total collection efficiency" values may also allow to

2 The "témoin CEV" is a bar-control developed and implemented by the French Test Flight

Centre -"Centre d'Essais en Vol"- and commonly adopted in France. Results are used by industry and research.

∆e h

ρ = ∆e / h

β = δe / δSδe

1-23

remedy the lack of accuracy.

This CEV reference is particularly interesting in relation to ATR aircraft. These aircraft are fitted with an "Ice Evidence Probe" mounted outside the left side cockpit just under the forward side window. The IEP has main comparable characteristics than the CEV reference in terms of shape and size (leading edge size of one inch (2.54 cm). The "total collection efficiency" ρ is also close to one, but it only represents the IEP itself and is not applicable to the aircraft profile, whose ρ is weaker.

1.16.4.2.2.3 Other researches

The NASA Icing Branch has developed a computer program, called LEWICE (LEWis ICE accretion program), to provide information about the ice accumulation and extend of ice coverage (impingement limit) that might have accreted on the airplane. It’s a software used by literally hundreds of users in the aeronautics community for predicting ice shapes, collections efficiencies, and anti-icing heat requirements. The atmospheric parameters of temperature, pressure, and velocity, and the meteorological parameters of liquid water content (LWC), droplet diameter, and relative humidity are specified and used to determine the shape of the ice accretion.

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1.18.1 Air Traffic Control

The operations of CKS Approach Control Tower and Taipei Area Control Center were normal. The controlling operations of GE791 were transferred to Taipei Area Control Center from CKS Approach Control Tower at 0121:30, Dec 21.

1-25

1.18.2 Radar

1.18.2.1 General Speaking

Airport Surveillance Radar (ASR) data were acquired from the Taipei Air Control Center (TACC) of CAA and Xiamen of China. There were five radar that detected the accident flight track including: CCC(CKS Radar), KSR（Kaohsiung Radar）, MKR（Makung Radar）, Dahansan, and Xiamen. Both Secondary Radar Returns and Primary Radar Returns were recorded in the various radar data sets. The MKR that close by the GE791 accident site only recorded the primary data.

After accident occurred, TACC provided all relevant radar data. Both primary and secondary radar data were extracted as Continuous Data Recording (CDR) or National Track Analysis Program (NTAP) text format. NTAP data format includes: time, beacon code of the airplane, altitude, longitude and latitude position. The data contents of CDR include: time, slant range, azimuth / ACP3s. The flight track, track angle and ground speed were calculated according to the location of radar site and PSR data.

Xiamen radar is a secondary radar system that only records the secondary signals. The system can only playback the recording with video format. Viewer transcribed the time, ground speed and Mode C altitude manually.

1.18.2.2 Secondary Radar Signals

These five radar’s Secondary Radar covered the accident flight track from taking off until signal disappeared on the radar screen. The timing system of radars are different, Makung radar timing system is selected as reference time to synchronize the others. Table 1.18-1 lists time correlation, scan rate, time duration of each radars data for the GE791, and relevant Mode-C altitude.

Figure 1.18-1 shows the GE791 radar track recording from 17:03:31UTC (100ft) until 17:52:49.129UTC. According to TACC radar recordings, the last transponder signal received from Makung radar was at 17:52:49UTC, and the altitude was 1,500ft. Figure 1.18-2 shows that GE791 started to descent at 17:52:04.780UTC. The last transponder signal received from Xiamen radar

3 ACP（Azimuth Change Pulses），Digitalize azimuth angle（0° to 360°）to 0 to 4095.Therefore,1

ACP=360°/4096 = 0.08789°。

1-26

was at 17:52:38UTC, the altitude was 2,740 m (8,989 ft). Fig. 1.18-3 shows the superposition of the GE791 radar tracks, which were detected by the MKR, CCC, and KSR.

Table1.18-1 Time correlation of each radar sites between MKR

Starting Time（UTC）Ending Time（UTC） Radar Site Scan Rate

Mode-C Altitude（Ft）Mode-C Altitude（Ft）

Time difference

between MKR

Radar Site (sec)

1747:11 17:52:10 TACC-NTAP 12 sec/times

18000 17900 0

1751:38 17:52:46 DAHAN –NTAP 12 sec/times

18000 3600 -0.2

1703:31.777 17:52:22.129 CCC-CDR 4.6 sec/times

100 16600 0

1709:58.78 17:52:49.129 MKR-CDR 5 sec/times

6800 1500 0

1750:04.130 17:52:34.582 KHR-CDR 4.6 sec/times

18000 11200 -0.56

1750:11 17:52:38 Xiamen-Radar

Image Record 4 sec/times

18011.59 2740m(8989ft) -4

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Figure1.18-1 Mode-C altitude of GE791 (17:03: 31~17:52:56UTC)

Figure1.18-2 Mode-C altitude of GE791 (17:51:38~17:52:48UTC)

1-28

Figure1.18-3 Superposition of the GE791 radar track, which were detected by the MKR, CCC, and KSR.

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1.18.2.3 Primary Radar Return

Primary radar returns extracted from the MKR were calculated, and correlated with the secondary radar returns. Figure 1.18-4 shows the results of superposed sonar targets. Figure 1.18-4 indicates the relative distance between the last transponder position (N23°28’47.89”, E119°26’23.04”, altitude 1500 ft) and major sonar targets is 186 meters. This figure also shows six primary radar returns were found near the accident site since 17:52:49UTC until 18:00:00UTC. Table 1.18-2 lists the primary radar returns with time, position, and the relative distance.

Table1.18-2 Primary radar return in the GE791 accident site

SSR last return time Latitude Longitude Distance (m) 17:52:49.129 119°26’23.04” 23°28’47.89”

No PSR return time Latitude Longitude Distance (m)1 17:52:49.129 119°26’21.33” 23°28’54.53” 206 2 17:52:54.130 119°26’37.32” 23°28’53.84” 442 3 17:52:59.000 119°26’39.29” 23°28’49.86” 463 4 17:53:04.000 119°26’30.67” 23°28’49.86” 225 5 17:53:13.776 119°26’37.32” 23°28’53.84” 442 6 17:53:58.453 119°26’27.72” 23°28’55.92” 281

Figure 1.18-4 Primary and Secondary radar return of MKR and sonar targets

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1.18.2.4 Radar Video Recording System of TACC/CAA

There are two radar data playback systems at TACC. One is the ATC Automation System (ATAS), which only records the secondary radar returns. The other is the Micro-ARTS, which can playback both primary and secondary returns. These radar video recordings were exported into the digital video recorder（DV） t, and post-processing the DV to specific frames. Figure 1.18-5 indicates the GE791radar track at 17:51:54.970UTC, its Mode-C altitude is 18,000 ft. The last transponder signal received from MKR at 17:52:49UTC, and then primary radar returns continuously appeared until 17:53:12UTC.

Figure1.18-5 Secondary and primary radar returns recorded by the ATAS（from TACC）

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IV. Attachments

Number Item 1-1 Meteorological Reports (METARs)

1-2 Terminal Aerodrome Forecasts (TAFs) for RCTP, VMMC, VHHH and RCQC

1-3 Significant Weather Charts (SIGWX Charts) issued from Taipei Aeronautical Meteorological Center, Hong Kong Observatory and Tokyo Aviation Weather Service Center

1-4 Significant Meteorological Information (SIGMETs) for Taipei FIR, Hong Kong CTA and Naha FIR

1-5 Doppler Weather Radar data of RCWF and RCCG from Central Weather Bureau

1-6 GMS-5 Infrared Satellite Imagery 1-7 Rain Amount Records from Central Weather Bureau 1-8 Radio soundings from Central Weather Bureau 1-9 Lightning Information of Taiwan Power Company

1-10 Weather Charts from Central Weather Bureau 1-11 Weather Information Provided To The Pilots

1-12 Data calculated and forecasted by MM5 Model from Taipei Aeronautical Meteorological Center

1-13 Winds calculated by the recorded parameters of GE791FDR

1-14 ICAO ANNEX 3 Meteorological Services for International Navigation

1-15 ICAO DOC 8896 Manual of Aeronautical Meteorological Practices

1-16 Operations Manual of Taipei Aeronautical Meteorological Center, CAA

1-17 Secondary Radar Data for the GE791 (NTAP format, TACC)

1-18 Secondary Radar Data for the GE791 (NTAP format, DAHAN)

1-19 Secondary Radar Data for the GE791 (CDR format, MKR)1-20 Secondary Radar Data for the GE791 (CDR format, CCC)1-21 Secondary Radar Data for the GE791 (CDR format, KSR)

1-22 Secondary Radar Data for the GE791 (Text format, Xiamen)

1-23 Air-Ground Communication Recording – 125.1mhz 1-24 Air-Ground Communication Recording – 126.7mhz

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1-25 Air-Ground Communication Transcripts – 125.1mhz 1-26 Air-Ground Communication Transcripts – 126.7mhz 1-27 Flight Plan 1-28 Related NOTAM(S)

1-33

V. Appendix

1-1 The Upper level wind and temperature data calculated from the MM5 Mesoscale Model

(1) 1700 UTC

”CHALI” ”CANDY” ”SIKOU” Altitude（FL） Temp.

（℃） Wind

（°/Kt）Temp.（℃）

Wind （°/Kt）

Temp.（℃）

Wind （°/Kt）

180 -11.1 250/24 -11.3 250/23 -10.7 240/25 170 -9.3 250/23 -9.6 250/22 -9.2 250/22 160 -7.4 250/21 -7.7 250/21 -7.7 250/20 150 -5.5 260/21 -5.7 260/20 -5.9 250/19 140 -3.6 260/20 -3.7 260/20 -4.0 260/19 130 -1.8 260/19 -1.7 260/19 -2.0 260/19 120 -0.4 260/18 -0.3 260/18 -0.4 260/18 110 0.5 250/17 0.7 260/17 0.9 260/17 100 2.2 240/18 2.3 250/17 2.5 260/16 90 4.2 230/18 4.2 240/17 4.3 250/15 80 6.4 220/18 6.4 230/18 6.4 240/15 70 8.8 210/18 8.8 220/18 8.8 230/15 60 11.2 200/16 11.2 210/17 11.3 220/15 50 12.9 200/11 13.1 210/12 13.4 210/11 40 13.9 150/03 14.0 200/04 14.5 210/06 30 14.7 050/09 14.9 040/07 15.2 010/02 20 16.1 040/17 16.3 020/15 16.2 010/09 10 17.8 030/18 17.9 010/16 18.0 360/14

1-34

(2) 1800 UTC

”CHALI” ”CANDY” ”SIKOU” ”MAKUNG” Altitude（FL） Temp.

（℃）

Wind （°/Kt）

Temp.（℃）

Temp.（℃）

Wind （°/Kt）

Temp.（℃）

Wind （°/Kt）

Temp.（℃）

180 -10.7 260/29 -10.4 260/28 -9.8 260/28 -10 240/29170 -9.0 270/26 -8.8 260/24 -8.3 260/23 -9 250/23160 -7.1 270/23 -7.2 270/20 -6.8 270/18 -7 260/17150 -5.2 280/21 -5.4 280/18 -5.1 270/14 -6 270/11140 -3.3 280/19 -3.5 280/16 -3.2 280/11 -4 280/09130 -1.3 280/16 -1.4 290/14 -1.3 290/10 -2 290/07120 0.2 280/13 0.3 290/11 0.5 300/08 0 290/05110 1.3 270/10 1.7 290/08 2.0 310/06 1 290/04100 2.7 260/08 3.1 280/06 3.5 310/04 3 300/0390 4.2 230/07 4.6 260/04 5.1 330/02 5 280/0280 6.1 210/08 6.4 220/04 6.9 010/01 7 330/0370 8.3 200/07 8.6 210/04 9.0 080/01 9 360/0460 10.4 170/03 10.8 190/02 11.3 130/01 11 010/0550 12.1 070/07 12.4 080/05 13.1 110/04 13 020/0540 13.5 060/14 13.9 070/11 14.3 090/08 15 040/0730 15.0 060/18 15.4 070/16 15.3 080/11 16 080/1020 16.5 060/19 16.9 060/16 16.7 070/12 17 080/1110 17.9 040/15 18.3 040/14 18.3 050/12 18 070/11

1-35

1-2 GMS-5 infrared satellite images at 1731 UTC

1-36

1-3 The SIGWX chart issued from TAMC for FL100-FL250 and was valid at 1800 UTC, Dec 20

1-37

The SIGWX chart issued from TAMC for FL100-FL250 and was valid at 0000 UTC, Dec 21

1-38

1-4 The SIGWX charts issued from HKO for FL100-FL250 and was valid at 1800 UTC

1-39

1-5 The SIGWX charts issued from TAWSC for SFC to 14,000 meters and was valid at 1800 UTC on Dec. 20 and 0000 UTC on Dec. 21

1-40

1-41

1-6 The PPI of radar images with the ground track of GE791 superimposed

1-42

1-43

1-44

1-45

1-46

1-47

1-48

1-7 The cross section chart of radar images with the track of GE791 superimposed

RCWF

RCCG

7

6

5

4

3 Km

1

2

0

7

6

5

4

3 Km

1

2

0 MAKUNCHALI CANDY SIKOU

MAKUNCHALI CANDY SIKOU

1-49

a. "Penn State University" diagram

1-50

1-8 "Lucas Aerospace" diagram

TAS (kt)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0 1.0 2.0 3.0 4.0 5.0

Ice Accretion Speed (mm/minute)

Liqu

id W

ater

Con

tent

(g/m

3 )

100

110

120

130

140

150

160

170

180

190

200

210

220

230

240

250

Theoretical liquid water content and ice accretion speed vs TAS (kt)