An EasyJet Boeing 737-700 dived 10,000ft (3,050m) during a 12 January post-maintenance test flight because a misunderstanding had led to the elevator balance tabs being adjusted in the opposite sense to that intended, according to the UK Air Accidents Investigation Branch.
As a result, when the crew switched off hydraulic power to the controls in accordance with the manual reversion flight-test procedure, the aircraft pitched down violently and the captain could not apply sufficient physical force to counter the manoeuvre.
The captain who carried out the test flight had delivered the aircraft to the maintenance provider at Southend airport a month before, and during the delivery flight he had noted that the amount of pitch trim required to keep the aircraft in level flight during manual reversion was within limits, but only just.
According to EasyJet, when the aircraft commander was speaking to the crew chief upon delivering the aircraft, he did not request specific action relative to the trim tab, he only pointed out that it was close to its allowable setting limits, but that since it was within those limitations he had not entered it in the technical log.
The interim AAIB report comments: "The absence of a formal post-flight debrief and formal written record resulted in the [elevator] balance tabs 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."
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© Chris Proctor/AirTeamImages.com |
After take-off from Southend for the post-maintenance test, the captain climbed the aircraft (G-EZJK) to flight level 410 (41,000ft) over the north sea in a military training area, where it was cleared to manoeuvre. Before carrying out the manual reversion test the 737 descended to level out at FL150 with an indicated airspeed of 250kt (460km/h).
According to procedure the crew switched off the autopilot and authothust, selected the stabiliser electrical trim to "cutout" and the spoilers to "off". Finally hydraulic power to the flight controls was isolated by setting the flight control switches A and B to "off", and it was at this point that the aircraft pitched down, and the captain could not limit the attitude to less than 2.81° nose low, which gave a descent rate of 3,100ft/min (15.74m/s).
The captain did not want to re-engage the hydraulics while applying back pressure, so he rolled left to 91.2° bank and says he released the controls before calling for the co-pilot to select hydraulic power back on for the controls, but a "misunderstanding" recorded on the cockpit voice recorder meant this did not take place, says the report.
The captain, thinking power was back on, rolled the wings level and attempted to arrest the rate of descent "which peaked at 21,000ft/min", with the nose down 30°, and the indicated airspeed at 440kt. The commander recovered the aircraft at 5,600ft in cloud, and the flight-control hydraulics were selected back on, restoring the control forces to normal levels.
The crew recovered the aircraft safely to Southend for checks.
EasyJet says it has suspended further check flights until it has reviewed its procedures for them, and the UK Civil Aviation Authority is reviewing its check flight handbook, saying it intends to publish an airworthiness communication "relating to the co-ordination between operators and maintenance organisation surrounding the conduct of maintenance check flights".
The AAIB says it is continuing the investigation and will publish a final report on this incident in due course.
Source: Flight International
