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Thursday, February 2, 2023

Incident: Qatar A350 Stall Warning Triggered By MD83 Autopilot Malfunction

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A Caspian Airlines McDonnell Douglas MD-83, registration EP-CAS, was en route from Tehran Mehrabad to Kish (Iran) at FL330 about 50nm northwest of Isfahan (Iran) when the autopilot failed, causing the aircraft to climb 400 feet.

A Qatar Airways Airbus A350-1000, registration A7-ANO, was en route at FL340 about 50nm northwest of Isfahan (Iran), when the crew got a TCAS resolution advisory to climb due to the MD-83 having climbed above their assigned altitude. The A350 gained at least 550 feet in altitude, and the crew received a speed/stall warning throughout the climb.

The MD-83 returned to their allocated flight level 330, while the A350 returned to FL340. Both aircraft resumed their flights to their destinations without a problem.

Iran’s CAO reported on April 23rd, 2021 that the MD-83 climbed around 400 feet above their assigned altitude due to an autopilot problem, resulting in a TCAS resolution alert. During the climb, the A350 received a speed/stall warning. The incident was classified as serious and is being examined by Iran’s AIB.

On September 1st, 2021, Iran’s AIB issued their final report (which is presently only available on the AVH website as the cao.ir website is still offline for the foreseeable future), concluding that the probable causes of the serious incident were:

The Aircraft Accident Investigation Board (AAIB) determined that the main cause of the serious incident for MD83 was an incorrect pilot reaction to a malfunction of the alternate trim motor control and excessive manual trimming of the aircraft, which caused level bust and generated TCAS RA for both aircraft.

The A350 cockpit crew did not follow FCTM recommendations and chose OP DES in a low energy condition, particularly at high altitude, resulting in a “Low Energy” warning.

According to the AIB, the MD-83 was dispatched with only the alternate trim motor control due to minimal equipment list limitations. The aircraft had been flying at FL330 when it lost 500 feet during a maneuver and was climbing back to FL330 when an alternate trim motor control relay malfunctioned, causing the aircraft to climb 400 feet above the cleared FL330.

The Qatar A350 was en route at FL340 when the autopilot first armed and then automatically started a TCAS climb in response to a TCAS resolution advisory to rise. The autopilot climbed the aircraft up to FL354, which was above the existing maximum altitude of 35100 feet.

Because the autopilot only observed the programmed climb rate, the airspeed decayed, resulting in a “LOW ENERGY” alert when the airspeed fell around 1 knot below VLS. The MD-83 was given a TCAS resolution advising her to descend. The MD-83 returned to FL330, and once TCAS reported that the issue had been resolved, the A350 descended to their assigned flight level 340.

The AIB provided the following description of the alternate trim motor control procedure:

1- Check that Manual Alternate Trim works properly by moving the Alternate Trim Switch Levers on the center pedestal and watching the longitudinal trim position indicator move.
2- Trim the airplane manually before utilizing the autopilot.
3- Keep an eye on the AP TRIM’ OUT OF TRIM light when the AP is turned on.
4- If the light comes on, re-trim by doing one of the following:
a. Using the alternate longitudinal trim switch (which does not require AP disengagement) or b. Disconnecting the autopilot, manually trimming with control wheel switches or LONG; TRIM handles and then re-engaging the AP.

The AIB investigated:

When the MD aircraft began turning, the aircraft flying level decreased due to a trim motor breakdown, therefore the pilot attempted to trim the elevator manually, resulting in a high rate of climb detected by the A350’s TCAS transponder, resulting in TCAS RA. The cockpit crew’s behavior demonstrates that they did not follow the above steps correctly.

Boeing thinks that while dispatching an MD-83 using MEL Item27-7 and the operating requirements of this MEL Item, as well as the related Dispatch Deviation Guide Item27-7, the following procedures should be followed:

– The airplane can be handled safely – The autopilot is not expected to allow an altitude loss during a heading change – A qualified pilot is expected to manually fly the airplane to regain 500 feet of altitude without an altitude/level bust

According to the Qatari pilot’s report, which has been confirmed by A350 FDR data, when the aircraft was cleared from other conflicted aircraft at FL351, by the time the “speed, speed” warning was heard in the cockpit and taking into account the maximum recommended altitude, the aircraft speed was reduced to the Lowest Selectable Speed (VLS) situation. In view of the A350’s gross weight (303.2T) and high ROC, the aircraft proceeded to climb and, after crossing FL354, when the aircraft speed dropped to M.792, the aircraft continued on an open descent to FL340. Finally, return to normal speed (M.85) and level off at FL340. According to data, the MD83 was at FL329 when the A350 passed through FL351.

According to BEA comments, the A350 aircraft’s performance was not degraded at any moment, but it was limited due to its gross weight and high altitude.

The low energy alarm resulted from the extended TCAS resolution alert, which required the Airbus to climb at 2500 ft/min while already close to REC MAX FL. The climb rate was reached, but the performance limitation at that altitude resulted in a temporary decrease of speed down below VLS. During a TCAS maneuver, where the priority is to accomplish the required vertical rates, speed loss is acceptable.

Source avherald

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