Thursday, March 05, 2009

Preliminary factual report on the easyJet B737-700 upset incident

easyJet Boeing 737-700Back in January of this year, a Boeing 737-700 aircraft operated by British carrier easyJet experienced a 'violent pitch down' during which it exceeded Vmo (maximum operating speed) by 100 knots, and dropped 10,000 feet. Fortunately the incident happened during a non-revenue flight, and no one was injured -- although I think it's safe to bet that it scared the bejeezus out of the four crew members on board.

Earlier this week, the UK's Air Accidents Investigation Branch (AAIB)  issued a preliminary factual report about this serious incident, in the form of a Special Bulletin.  From that report we learn that the aircraft, which was at the end of its lease, had just undergone maintenance, prior to it being handed over to another operator. The incident flight was a combined maintenance check and customer demonstration flight, designed to confirm the aircraft’s serviceability.

The incident occurred during a flight control manual reversion check at FL150. Here is what happened, according to the AAIB report (reparagraphed for easier reading):
This required the aircraft to be flown at FL150, at 250 kt IAS with the fuel balanced, the AUTOPILOT and AUTOTHRUST selected OFF, the STAB TRIM MAIN ELEC and AUTOPILOT switches set to CUTOUT and the aircraft in trim.

The ‘customer demonstration flight schedule’ also required SPOILER A and B switches to be selected OFF. All these checks were conducted using the operator’s ‘customer demonstration flight schedule’ and not the maintenance manual extracts as the guiding reference.

Before the manual reversion check commenced, the individual hydraulic systems were isolated by placing the FLT CONTROL switches A and B to the OFF position individually and reinstating in turn enabling the flight controls to be checked for normal operation on a single hydraulic system. Operation was confirmed as satisfactory on both systems.

Then, with the commander having released the controls, the co-pilot selected FLT CONTROL switches A and B to the OFF position, removing all hydraulic assistance from the primary flying controls. As he did so the aircraft suddenly pitched nose down.

The commander pulled back on the control column with considerable force but was unable to prevent the aircraft from maintaining a nose down pitch attitude of ‑2.81° and descending at up to 3,100 fpm. The commander, therefore, decided to abandon the check but did not wish to re-engage the hydraulics whilst applying significant backpressure to the controls.

The commander stated that, should the aircraft pitch up or down uncontrollably during a manual reversion check, he had been trained to roll the aircraft to unload the pressure on the elevator and release the controls before reinstating the hydraulics. The commander therefore, rolled the aircraft left 91.2° and believes he released the controls before calling for the co-pilot to re-engage the FLT CONTROL switches.

The recording from the Cockpit Voice Recording (CVR) indicated that at this point there was confusion between the two pilots. This resulted in the commander thinking that hydraulic power had been restored to the flight controls although there is no evidence that the FLT CONTROL switches had been moved from the OFF position.

The commander rolled the wings level and attempted to arrest the rate of descent which had increased considerably, peaking at 21,000 fpm; the aircraft had pitched 30° nose down after the aircraft had been rolled to the left.

The control forces remained high but the commander considered this to be due to the aircraft’s speed, which both pilots observed to be indicating above 440 kt. He retarded the thrust levers and selected the speed brakes, however, the spoilers had been switched OFF as part of the test procedure.

The commander continued to maintain backpressure on the controls and made a PAN call to ATC. The aircraft eventually recovered from the dive at about 5,600 ft, having entered a layer of cloud. The pilots reviewed the situation and selected the FLT CONTROL switches, which had remained OFF throughout the flight excursion, to the ON position. The control forces returned to normal.
.. and shortly thereafter they landed that puppy back at Southend, where it had originated, without further incident. (One can only imagine the state of their underlinens!)

So why did this dramatic excursion happen? Since the purpose of this AAIB report was to provide preliminary factual information, not analysis, there was no 'probable cause' stated. Nevertheless, there are some clues to where the ongoing investigation may be headed.

The pilot in command during the incident had earlier ferried the aircraft to the contract maintenance facility. During that earlier ferry flight, a 'shakedown' test was performed to identify any existing defects so that they could be brought to the attention of the maintenance provider, and rectified.

One of the items in the 'shakedown' was the manual reversion test to assess the trim of the aircraft. Quoting again from the AAIB report:
...This involved switching off both hydraulic systems powering the aircraft flight controls and assessing the amount of manual stabiliser trim wheel adjustment required to balance the aircraft in level flight.

The results of this test identified that the aircraft was within, but very close to, the approved maintenance manual limits.

Following the flight, the commander verbally requested that this be addressed during the subsequent maintenance input, but elected not to enter it in the tech log, as the level of stabiliser trim required during the test had been within limits.

The absence of a formal post‑flight debrief and formal written record resulted in the balance tabs, attached to the elevators of the aircraft, being adjusted in the opposite sense to that identified as necessary by the flight test. The aircraft was therefore significantly out of trim during the post-maintenance test flight, and it was that which initiated the pitch-down incident during the manual reversion test.
Here is the link to the AAIB report: AAIB Special Bulletin S2/2009 - 4-page 'pdf' file