QR739 - Incident: Caspian MD83 and Qatar A35K near Isfahan on Apr 12th 2021

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

A Qatar Airways Airbus A350-1000, registration A7-ANO performing flight QR-739 from Doha (Qatar) to Los Angeles,CA (USA), was enroute at FL340 about 50nm northwest of Isfahan (Iran), when the crew received a TCAS resolution advisory to climb due to the MD-83 having climbed above their assigned altitude. The A350 climbed at least 550 feet, the crew received a speed/stall warning during the climb.

The MD-83 descended back to their assigned flight level 330, the A350 also descended back to FL340, both aircraft continued their flights to destinations without further incidents.

On Apr 23rd 2021 Iran's CAO reported that the MD-83 climbed about 400 feet above their assigned altitude due to an autopilot malfunction, which caused a TCAS resolution advisory. The A350 received a speed/stall warning during the climb. The occurrence was rated a serious incident and is being investigated by Iran's AIB.

On Sep 1st 2021 Iran's AIB released their final report (currently available only via the AVH website as the cao.ir website is currently offline for unforeseeable future) concluding the probable causes of the serious incident were:

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

Cockpit crew of A350 did not follow FCTM recommendation and selected OP DES in a low energy situation, and especially at high altitude that caused “Low Energy” warning.

The AIB reported the MD-83 had been dispatched under minimum equipment list requirements with only the alternate trim motor control. The aircraft had been enroute at FL330, but lost 500 feet during a turn and was climbing back to FL330 when an alternate trim motor control relay malfunction occurred causing the aircraft to climb above the cleared FL330 by about 400 feet. The Qatar A350 was enroute at FL340 when the autopilot first armed and subsequently automatically commenced a TCAS climb in response to a TCAS resolution advisory to climb, the autopilot climbed the aircraft up to FL354 above the current maximum altitude of 35100 feet. As the autopilot only observed the commanded climb rate to be achieved, the airspeed decayed resulting in a "LOW ENERGY" warning when the airspeed dropped about 1 knot below VLS. The MD-83 received a TCAS resolution advisory to descend. The MD-83 returned to FL330, after TCAS reported clear of conflict the A350 also descended back to their assigned flight level 340.

The AIB described the alternate trim motor control procedure:

1- Verify Manual Alternate Trim functions properly by moving Alternate Trim Switch Levers on the center pedestal and observing movement of the longitudinal trim position indicator.
2- Trim aircraft manually prior to engaging autopilot.
3- Monitor AP TRIM' OUT OF TRIM light while AP is engaged.
4- If light comes on, re trim by either of the following:
a. Operation of the alternate longitudinal trim switch (which does not require AP disconnect)
Or
b. Disconnecting the autopilot, manually trimming with control wheel switches or LONG; TRIM handles, and then re -engaging the AP.

The AIB analysed:

When MD aircraft initiated turning, the aircraft flight level decreased due to malfunction of trim motor so pilot tried to trim elevator manually that caused high rate of climb which was sensed by TCAS transponder of A350 resulting TCAS RA finally. Results of the of the cockpit crew reaction shows they did not follow above procedures correctly.

Boeing believes, however, that when dispatching an MD-83 utilizing MEL Item27-7 and the operations requirements of this MEL Item, And the associated Dispatch Deviation Guide Item27-7, are followed:

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

In accordance with Qatari pilot’s report which has confirmed by A350 FDR data, when the aircraft was cleared from other conflicted aircraft at FL351, by the time “speed, speed” warning was heard in the cockpit and with consideration of maximum recommended altitude, aircraft speed reduced to Lowest Selectable Speed (VLS) situation. Regarding to the A350 gross weight (303.2T) and high ROC, the aircraft continued to climb and after passing FL354 finally when the aircraft speed reduced to M.792, the aircraft continued for open descent to FL340. Finally, aircraft speed back to normal speed (M.85) and level off at FL340. information shows that when the A350 was passing FL351, the MD83 was at FL329.

In accordance with BEA comments, the performance of the A350 aircraft was not degraded at any time but performance was limited due its gross weight and high altitude.

The low energy alert was a consequence of the extended TCAS resolution alert requiring the Airbus to climb at 2500 ft/min while it was already close to REC MAX FL. The climb rate was achieved but the performance limitation at that altitude resulted in a loss of speed, transiently to below VLS. Loss of speed is acceptable during a TCAS manoeuver, where the priority is to achieve the required vertical rates.

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