In January this year an EasyJet Boeing 737-700 dived 10,000ft (3,050m) before being brought back under control during a post-maintenance test flight. In November last year an Air New Zealand-owned Airbus A320, also undergoing a check flight following maintenance, crashed into the sea off the French Mediterranean coast, killing the crew.
According to the UK Air Accidents Investigation Branch report, the EasyJet incident occurred because of a misunderstanding between the operator and the maintenance and repair organisation. As a result, the elevator balance tabs were adjusted in the opposite sense to that intended, says the AAIB.
So 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 same aircraft to the maintenance provider at London Southend airport a month before. 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 spoke to the crew chief upon delivering the aircraft to the MRO, he did not request specific action relative to the trim tab, merely pointing out that it was close to its allowable setting limits, but since it was within those limitations he had not entered it in the technical log.
The 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."
The maintenance was being carried out because the 737 had reached the end of its lease contract with EasyJet and it had been sent for specific maintenance to comply with contractual requirements.
Maintenance confusion almost destroyed an Easyjet 737-700
EasyJet says things have changed since then: "Following the test flight on 12 January, EasyJet suspended all further checks on the aircraft and instigated a full investigation into the circumstances of the flight and the associated engineering procedures.
"As a result of that investigation, a number of amendments were made in respect of the engineering interface and flight-test schedules. The resultant changes were effected immediately. The event is still under investigation by AAIB."
The "engineering interface" particularly worried the UK Civil Aviation Authority. Since that time it has been reviewing its check flight handbook to ensure the specific guidance related to flying control checks is not open to misinterpretation.
The CAA has also published a flight operations directive clarifying the issues relating to the co-ordination between operators and maintenance organisations about maintenance check flights.
This says: "Operators and maintenance organisations should review their guidance to staff involved in the planning of, preparation for, execution of and follow-up to check flights". It is highly specific in what it requires.
Meanwhile, the European Aviation Safety Agency says it is concerned about both the A320 Perpignan accident and the EasyJet 737 serious incident. In June EASA, French accident investigation agency BEA, and the AAIB met to discuss protocols for various types of post-maintenance check flights.
The BEA observes in its interim factual report that the crew of the crashed A320 did not appear to be adhering to a pre-agreed check flight schedule, and had failed to make appropriate arrangements with air traffic control for the use of suitable airspace in which to carry out the necessary tests.
The aircraft was crewed by pilots and engineers from XL Airways Germany and Air New Zealand. XL was the aircraft operator whose two-year lease of the A320 was ending, and ANZ - the owner - was carrying out acceptance checks before receiving it back.
EASA explains that, in accordance with EU-Ops, each operator should have defined the various types of non-revenue flights in their operating manual, including the procedures and processes to be followed.
As this is part of the documentation set required for their air operator's certificate, EASA says it should be reviewed and monitored by the competent national aviation authority. But the agency knows regulatory change is needed, and adds: "We should look at this as a need for the NAA to take short-term action and EASA to take long-term action."
EASA acknowledged receipt of a number of safety recommendations from the BEA in early July. These included that "EASA detail in the EU-Ops the various types of non-revenue flights that an operator from an EU state is authorised to perform.
The BEA recommends that EASA require that non-revenue flights be described precisely in the approved parts of the operations manual, this description specifically determining their preparation, programme and operational framework as well as the qualifications and training of crews."
The agency puts the recommendations in context: "These safety recommendations have implications for flight standards rulemaking. The safety recommendations are to be the subject of a preliminary risk impact assessment within the scope of future operating rules. We note that the UK Civil Aviation Authority has issued an Aircom on the subject of 'Ensuring Satisfactory Co-ordination between Operators and Maintenance Organisations for Maintenance Check Flights'.'
The BEA interim report about the A320 accident says: "The crew had not received any specific training for this type of flight. The Air New Zealand pilot had undertaken two simulator training sessions following the programme described in by the operational flight check document." The BEA says existing guidance requires that the crew [should] receive a full briefing on the flight schedule, to ensure that the objective and conditions associated with this type of flight are clearly understood.
The purpose of the flight was to check the aircraft's condition at the end of a leasing agreement, but the BEA comments that this type of flight is not included in the list of non-revenue flights detailed in EU-Ops.
The BEA says it has been unable to identify any text that sets a framework generally for non-revenue flights or specifically for "acceptance" flights. The agency observes that the majority of operators appear to base acceptance flight routines on the check flights performed after maintenance. Before the accident flight, ANZ and XL Airways Germany had agreed on a programme of in-flight checks based on an Airbus programme used for customer acceptance flights for new aircraft.
The investigation found that there is considerable diversity in operators' definitions of non-revenue flights, in arrangements for their preparation and execution, and in the selection and training of pilots. The BEA says this diversity, along with the lack of guidance on standards for non-revenue flights, tends to cause crews to improvise test routines.
Meanwhile, in the USA, concern about non-revenue flights has also been growing. On 12 October 2008 the US Federal Aviation Administration issued a safety alert recommending that operators should analyse data from flight data recorders after all non-revenue flights to identify any deviations from procedure.
The US National Transportation Safety Board determined that, in the past 10 years, 25% of accidents to turbine aircraft occurred during non-revenue flights, and noted that there were two common contributory factors for all these accidents: failure to respect standard operating procedures and/or a failure to respect the aircraft's limitations.
It seems MROs and operators should talk more before check flights, and keep records of everything. If they do not, the regulators warn, misunderstandings can happen.