Maintenance Check Flights: Safety Lessons

We look at the safety lessons from three UK Air Accidents Investigation Branch (AAIB) reports into serious incidents during maintenance test flights (the most recent published this month).

CL600 N664D ADG Check Flight March 2014

In March 2014 the US registered Bombardier CL600-2B16 required a post-maintenance check flight at Biggin Hill, UK.  The check involved deploying the aircraft’s air-driven generator (ADG) and therefore placing the aircraft in an emergency electrical configuration. During the flight, with two crew accompanied by an engineer as an observer, the required check was successfully completed.  However, the aircraft was not returned to the normal electrical configuration. As a result the flaps, ground spoilers, anti-skid and nosewheel steering remained disabled. Consequently, on landing, with difficulty the aircraft was stopped approximately 120m from end of the runway (the Landing Distance Available was 1,550m).  All four main tyres deflated, causing damage to the left hand wheel and brakes.  The AAIB note that the CVR were not promptly deactivated to preserve the recording. The AAIB report highlights a difference in understanding between the flight crew and engineering personnel:

The commander observed that the roles of the pilots and the engineer had not been clearly established before takeoff, and the pilots assumed that the engineer would ‘talk through’ what he needed to see once airborne. The commander also observed that the crew should have referred to the Quick Reference Handbook (QRH) for the situation rather than rely on the engineer to guide them. The commander later discovered that the engineer was not expecting to make decisions or inputs during the flight, so a misunderstanding had existed.

The engineer described his role as primarily that of an observer, although with the intention of noting any defects or abnormalities that might arise during the flight test.

The maintenance organisation internal investigation made several recommendations including:

…the requirement for a full briefing to be given to flight crews undertaking a maintenance check flight, irrespective of whether or not the crew declared themselves to be familiar with the procedure.

Falcon 2000 CS-DFE Brake Fault Finding November 2009

In November 2009, flight crew were requested by maintenance to carry out high speed taxi trials of a Portuguese registered Dassault Falcon 2000 as part of the troubleshooting process of a braking defect, also at Biggin Hill, UK. Seven accelerate/stop runs were conducted along the main runway, at gradually increasing reject speeds. At the start of the eighth run, the crew felt that a tyre had deflated and brought the aircraft to a stop. ATC radioed that there was a fire under the left wing and all six persons on board safely abandoned the aircraft via the airstair door.  The Airport Fire and Rescue Service (AFRS) extinguished the fire.  The fire was caused by damage to the brakes from the excessive temperature of the repeated runs, this released hydraulic fluid under pressure, which then ignited. AAIB note that:

There were numerous systemic factors relating to the manner in which the operator conducted this test activity which contributed to the incident. Many of these, such as appropriate crew selection, the need for an approved test schedule and a detailed brief and debrief of the test activity with all involved personnel, are common to other recent incidents and accidents involving operators conducting maintenance or customer demonstration check flights. These issues have been highlighted and analysed in detail in the AAIB report into a serious incident involving a B737, G-EZJK, in the UK (reference: EW/C2009/01/02 AAIB Bulletin 9/2010) and a Bureau d’Enquêtes et d’Analyses (BEA) report into a fatal accident involving an A320, D-AXLA, in Perpignan, France (Report d-la081127).

B737 G-EZJK Post-Maintenance Check Flight April 2009

In January 2009 a B737 was undergoing a post-maintenance check flight over East Anglia, UK prior to being returned to its lessors. During a flight control check the aircraft pitched rapidly nose-down, descending approximately 9,000ft before control was recovered. A number of issues relating to the planning of the check flight, liaison with engineering and the involvement of multiple organisations emerged. These resulted in a flight control adjustment having been applied in the opposite sense to that intended after a prior check flight. Seven safety recommendations were made by AAIB. A few months after this serious incident the UK Civil Aviation Authority (CAA) issued a FODCOM on the subject of check flights.

Flight Safety Foundation Initiative

The Flight Safety Foundation (FSF) with Airbus, Boeing, Bombardier and Embraer, organized symposium in Vancouver in 2011 to discuss the challenges to be addressed and current best practices for conducting check flights.  They have since issued a comprehensive compendium of advice.


In many ways all three serious incidents highlight the importance of having clear procedures, well understood roles and responsibilities, good planning and briefing and appropriate training for what is often a relatively rare activity for most organisations that is higher risk than normal.

The most recent event in particular also demonstrate how vital it is to learn from the past experience of others as well as from your own incidents.

UPDATE 3o March 2015: EASA have issued NPA 2015-05 on Non-commercial operations of aircraft listed in the Operations Specifications (OpSpecs) by an AOC holder.  Among the types of operations discussed are maintenance test flights.

From the Executive Summary of the NPA:

This NPA aims to facilitate the risk analysis of non-commercial operations performed by AOC holders under the provisions of ORO.AOC.125; it proposes a minimum list of elements to be considered in the risk assessment process when the AOC holders follow different operational procedures from the ones normally used for their CAT operations. AOC holders will thus have flexibility in establishing operational procedures commensurate with the level of risk of a certain type of non-commercial operation.

The NPA also puts forward a list of non-commercial flights in order to standardise the various names used by industry to identify, sometimes, the same (or similar) types of flights.

UPDATE 28 June 2016: See also this article from Business and Commercial Aviation: Post-Maintenance Test Flying

Conducting post-maintenance flight checks safely requires pilots experienced in the aircraft, intensive preparation and cross-communication with the maintenance provider, and adherence to risk management principles

UPDATE 12 September 2016: The UK CAA issue an Information Notice on the selection / qualification of pilots for check flights.

UPDATE 29 October 2016: An EASA Opinion is expected in Q1 2017.

UPDATE 8 March 2017: EASA Opinion 01/2017 is released.

The objective of this Opinion is to mitigate the risks linked to maintenance check flights (MCFs). In MCFs, the pilots check the adequate functioning of aircraft systems that cannot be fully tested on the ground. This Opinion proposes to establish safety requirements to adequately select pilots and apply procedures for MCFs while distinguishing between MCFs with complex aircraft and MCFs with non-complex aircraft. The proposed changes are expected to increase safety of MCFs. Operators conducting the higher risk category of these MCFs with complex aircraft will have to develop their own procedures and ensure coordination between the operation, the continuing airworthiness management organisation (CAMO) and the involved maintenance organisation.

Other Flight Test / Check Flight Safety Resources


Aerossurance is pleased to be both sponsoring and presenting at a Royal Aeronautical Society (RAeSHuman Factors Group: Engineering seminar Maintenance Error: Are we learning? to be held on 9 May 2019 at Cranfield University.

raes are we learning

For practical specialist advice on airworthiness, check flight & flight test safety you can trust, contact Aerossurance at: