Cardig B733 at Wamena on Aug 28th 2015, main gear collapse on landing short of runway

Last Update: July 1, 2016 / 18:49:42 GMT/Zulu time

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Incident Facts

Date of incident
Aug 28, 2015

Classification
Accident

Aircraft Registration
PK-BBY

Aircraft Type
Boeing 737-300

ICAO Type Designator
B733

A Cardig Air Cargo Boeing 737-300 freighter, registration PK-BBY performing freight flight 8F-197 from Jayapura to Wamena (Indonesia) with 2 crew and 14 tons of rice, landed on Wamena's runway 15 at about 15:00L (06:00Z) but suffered the collapse of the left hand main gear on touchdown. The aircraft skidded along the runway before coming to a stop about 1500 meters down the runway just past the new apron.

The weather at the time was reported CAVOK with gusting winds from 150 degrees at 20-25 knots.

Authorities report the aircraft landed in strong winds causing the left main gear to collapse. The aircraft veered left and came to a stop at the left edge of the runway.

Ground witnesses reported the aircraft performed a visual approach and touched down at the threshold of the runway before the numbers. The aircraft touched down hard with the left hand main gear first, the gear collapsed, the aircraft subsequently skidded along the runway on its left engine pod, right main gear and nose gear. Boeing 737s always aim to touch down early in Wamena, just past the runway threshold.

On Aug 29th 2015 a 35 ton jack was flown in from Jayapura to lift the left engine onto a baggage cart, a sling was mounted at the left main landing gear support beam connected to a front loader to keep the wing up, another front loader was connected to a tow bar to tow the aircraft, and the aircraft was towed off the runway that way in the afternoon of Aug 29th 2015.

Wamena features a runway 15/33 of 2200 meters/7200 feet length, elevation 5100 feet/1550 meters.

On Oct 2nd 2015 Indonesia's NTSC released their preliminary report reporting the aircraft touched down 35 meters ahead of the runway threshold at a vertical acceleration of +3.68G and came to a stop about 1500 meters past the runway threshold with the left hand main gear collapsed and the left hand engine contacting the runway surface.

The aircraft received substantial damage: the left hand main gear collapsed with the trunion link fractured, the right main gear's shock absorber leaked and deflated, the left engine's thrust reverser received inner damage and engine housing received damage, the left wing's trailing edge flaps were dented, outboard and inboard flap fairings were dented, the flap transmission assembly #2 was broken.

As result of the preliminary report the NTSC recommended to install VASI on runway 15 amongst several other recommendations to the airline to monitor flight crew performance, restate that a go around must be initiated if the approach becomes unstable, to the airport and Directorate of Civil Aviation to ensure an adequate airport maintenance programme ...

On Jul 1st 2016 the NTSC released their final report concluding the probable cause of the accident was:

The large thrust reduction during the windshear resulted in rapid descend and the aircraft touched down with 3.683 G then collapsed the landing gear that had strength degradation.

The NTSC analysed:

The BMKG weather report was wind 150°/14-19 knots, visibility 10 km, cloud scattered cumulus and present weather nil. The Wamena Tower controller reported to the pilot that the wind was 150°/15 knots. The information of gust wind, which indicated the possibility of windshear, was not reported to the pilot.

The CVR recorded that during final approach at approximately 5,500 feet pressure altitude (300 feet AGL), the EGPWS caution “CAUTION WINDSHEAR” active. The FDR recorded that the head wind changed from 19 knots to 25 knots. This head wind changed met the criteria of the EGPWS mode 7 to trigger the caution. This was an indication that the windshear was exist.

The Boeing FCOM stated:

- If the autothrottle is disengaged, or is planned to be disengaged prior to landing, add an appropriate airspeed correction (correction applied in the same manner as gust), up to a maximum of 20 knots
- Avoid large thrust reductions or trim changes in response to sudden airspeed increases as these may be followed by airspeed decreases

The Boeing FCTM stated the recommended method for approach speed correction is to add one half of the reported steady headwind component plus the full gust increment above the steady wind to the reference speed. While the Boeing FCOM stated that for the windshear precaution, the maximum approach speed correction is 20 knots.

Refer to the CVR data, the reported head wind 15 knots and the approach speed correction should be 8 knots.

The VRef of this flight for the particular configuration and aircraft weight was 133 knots. The recommended approach speed was VRef + 5 or 138. The approach speed correction for head wind precaution was additional 8 knots, hence the approach speed should be 146. FDR recorded that approach speed was average at 150 knots.

The CVR did not record any pilot conversation related to the wind correction of windshear precaution following “CAUTION WINDSHEAR” activation.

The Boeing FCOM for the windshear precaution on approach and landing recommend: "avoid large thrust reductions or trim changes in response to sudden airspeed increases as these may be followed by airspeed decreases." The FDR recorded 6 seconds prior to touchdown, the N1 decreased from 71% to 41% followed by decreasing of airspeed from 151 knots to 129 knots. The vertical speed indicated rapid descend up to 1,320 feet/ minute.

The absence of speed correction following the information of headwind of 15 knots and pilot crew briefing after activation of EGPWS caution windshear indicated that the pilot did not aware of the existing windshear, that might be contributed by the absence of gust wind information.

The large thrust reduction was not in accordance with the FCOM for windshear precaution and resulted in rapid descend.

The NTSC analysed that 170 landings in 107 flights hours were present on the flight data recorder, the average vertical acceleration on landing had been +1.5G, in Wamena however +1.7G and there had been three landings (including the accident flight) exceeding +2.1G vertical accelerations.

A hard landing inspection should have been conducted at landings experienced +2.1G or more. The NTSC concluded analysis: "Based on the FDR data, the aircraft had received vertical acceleration of more than 1.5 G at the last 170 landings including two landings exceeded 2.1 G. The accumulation of such value of vertical acceleration might lead to landing gear strength degradation. The accident flight collapsed the landing gear, the FDR recorded the vertical acceleration was 3.683 G which was within the landing gear design limit. This indicated the degradation of landing gear strength."
Incident Facts

Date of incident
Aug 28, 2015

Classification
Accident

Aircraft Registration
PK-BBY

Aircraft Type
Boeing 737-300

ICAO Type Designator
B733

This article is published under license from Avherald.com. © of text by Avherald.com.
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