Mahan A306 near Shiraz on Jan 19th 2011, left pitot failure, pilot inputs cause 4.31G pulled in flight
Last Update: November 17, 2017 / 19:39:31 GMT/Zulu time
Iran's CAO released their final report concluding the probable causes of the serious incident were:
The cause of Mahan Air A300-600, EP-MNH incident is an uncontrolled action by the pilot on controls, responding to an unreliable airspeed fault on ADC1.The crew did not follow up the unreliable airspeed procedure according to the operations manual.
- Weak cockpit resource management.
- Technical defects on #1 Pitot heating system.
The CAO reported that following the flight the aircraft was grounded due to the high G-Loads of up to 4.31G experienced in flight. A subsequent inspection of the airframe did not reveal any damage or anomaly, however, Airbus advised Mahan Air to replace the trimmable horizontal stabilizer, which the airline complied with.
The CAO reported significant weather was forecast near Shiraz with thunderstorms up to FL180 and icing conditions between FL110 and FL180.
The cockpit voice recorder was overwritten until the time of landing. The flight data recorder was downloaded and revealed, that the unreliable airspeed event began when the aircraft climbed through 15,840 feet with the IAS reducing from 271 KIAS to 191 KIAS, while the speed over ground remained at 345 knots. Autothrust accelerated the engines from 96 to 106% N1. At 16,372 feet MSL a large nose down input was recorded, autopilot 2 disconnected, the pitch changed from 7 degrees nose up to 2 degrees nose down, autothrust was disconnected, the IAS further decreased to 33 KIAS with the speed over ground still being at 345 knots, the speed over ground began to increase and reached 414 knots, the engines reduced from about 105% to 93% N1, the stab trim changed from +2.6 degrees to +0.4 degrees, the pitch reached 18 degrees nose down, ground speed 411 knots, the pitch decreased further to 27 degrees nose down, the EGPWS issued a warning. A large nose up input was recorded, the engines accelerated to 106/110% N1, at 12,130 feet MSL, 415 knots over ground, the aircraft encountered +4.31G and began to climb again. 18 seconds later the aircraft levelled off at FL150 at 338 KIAS and continued the flight to Tehran within normal operating limits.
A post flight examination of the left pitot tube showed, that the insulation between heater resistance and pitot case was breached causing an intermittent continuity between heater resistance and pitot case. Both Air Data Computers were examined as well with no findings.
The CAO analysed that the overspeed warning was legitimate and obviously originated from ADC#2 (right hand pitot system) while the ADC#1 showed 33 knots IAS. The overspeed warning would be triggered if either of the ADCs exceeds 339 KIAS. It thus also followed that ADC#2 data including IAS and TAS remained valid throughout the event, and the IAS recorded on the flight data recorder came from ADC#1.
The CAO analysed that the autopilot 2 dropped offline as result of a large nose down input.
The CAO analysed that the autothrust disconnects if the IAS exceeds 511 knots or becomes invalid. Autothrust receives its input from ADC#1 even if autopilot 2 is engaged. Autothrust therefore disengaged due to invalid airspeed (from ADC#1).
The CAO further analysed the pitch trim was caused by manual crew input.
Due to the pitch trim and the elevator inputs the trimmable horizontal stabilizer produced a torque moment that reached 173% of its design limit. The fuselage loads as well as the engine loads remained near their design limits, the engine pylon loads remained below their design limits.
This article is published under license from Avherald.com. © of text by Avherald.com.
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