Breaking the Chain: X-31 Lessons Learned
On 19 January 1995 an X-31 experimental aircraft was making a fairly routine test flight from the NASA Dryden Flight Research Centre (now known as the the Armstrong Flight Research Centre) at Edwards Air Force Base. Suddenly the chase plane pilot makes the following radio calls:
NASA One, We have an ejection!! We have an ejection!! The aircraft is descending over the North base area… I have a chute… The pilot’s out of the seat and the chute is good…
This accident is valuable accident case study, particularly because NASA produced a candid video on the accident and its lessons.
The X-31 Enhanced Fighter Maneuverability Programme
The programme consisted of two Rockwell-Messerschmitt-Bölkow-Blohm X-31 Enhanced Fighter Maneuverability (EFM) aircraft, the first of which made its first flight in 1990. Rockwell is now part of Boeing and MBB part of Airbus. The aim of the X-31 was to demonstrate the value of in-flight thrust vectoring coupled with advanced flight control systems, to allow close-in air combat at very high angles of attack. Three thrust vectoring paddles were used to direct the exhaust flow to provide control in pitch and yaw. Movable forward canards and (eventually) with fixed aft strakes were fitted to further enhance pitch control. The X-31 was the only US X-plane programme that was an international collaborative effort (between the US and Germany). It also unique in that it supported two separate test programs during its life, one run largely by NASA and the other by the US Navy.
The X-31 Accident
The loss of the first X-31 (BuNo 164584) came at the end of a highly successful NASA test campaign. The flight was fitted with a different pitot tube (a Kiel probe), one that was unheated (as were research F-104 probes previously). The 43 minute flight was nearing its conclusion when ice began forming in the pitot tube. This caused erroneous airspeed data to be sent to the aircraft’s flight control computers. The result was a series of sudden, uncontrollable oscillations in all axes. As the aircraft pitched to 90 degrees angle of attack German pilot Karl-Heinz Lang was forced to eject.
NASA subsequently noted that:
Because the experimental pitot tube did not have a functional heater, X-31 mission rules prohibited flights in any type of precipitation or in the clouds. They did not, however, include a separate stipulation specifically prohibiting flight during potential icing conditions, despite simulations that had showed icing of the pitot static system could lead to loss of control under certain circumstances.
Additionally, the experimental probe installed on the X-31 was more susceptible to icing – a fact discovered through wind tunnel research performed after the accident. Information had been distributed among crew and engineers explaining the pitot tube change, but no formal closed-loop system (an internal system of checks and balances) had been in place to ensure that everyone had read and understood the change – in retrospect, a critical lapse in communication procedures.
The second X-31 went on to fly at that summer’s Paris Air Show and continued flying until 2003 in subsequent test programmes. The more details on the programme see: Flying Beyond the Stall
Sharing The Lessons
After the accident, NASA produced a candid 39 minute documentary that reviewed the accident. This is now available online: https://www.youtube.com/watch?v=x1E3xpePbmA Former Center Director Ken Szalai said in the opening remarks of one 2004 safety workshop at Dryden after the accident:
Many people involved had the opportunity to break the chain of events that led to the mishap. I was in charge. I cannot blame anyone else. But lessons from that might be applied to current projects.
Noting that the X-31 team was a highly skilled and experienced one:
If it happened to them, it can happen to you.
He also noted that warning signs are far easier to recognise in retrospect (known as hindsight bias). Dryden X-31 chief engineer John Bosworth comments in the video:
Prepare for the Unexpected – Expect to be Unprepared
Aerossurance has subsequently used this case study video many times over a number of years to facilitate successful safety workshops in multiple locations internationally.
Other Flight Test Safety Resources
- Fatigued Flight Test Crew Crosswind Accident
- AC-130J Prototype Written-Off After Flight Test LOC-I Overstress
- Maintenance Check Flights: Safety Lessons
- ANSV Issue AW609 Tilt Rotor Accident Investigation Update
- Bell 525 Prototype N525TA Fatal Flight Test Accident
- UPDATE 20 August 2017: 1980 MD-81 Flight Test Accident Video
Aerossurance is pleased to be supporting the annual Chartered Institute of Ergonomics & Human Factors’ (CIEHF) Human Factors in Aviation Safety Conference for the third year running. This year the conference takes place 13 to 14 November 2017 at the Hilton London Gatwick Airport, UK with the theme: How do we improve human performance in today’s aviation business?