Motorway Plane Crash ia the 16th episode of Seconds from Disaster investigates how two engines apparently failed at the same time.
British Midland Flight 092, a two-month-old Boeing 737-400, takes-off from Heathrow Airport in London, heading for Belfast. While en route the aircraft's left engine suffers a fan blade failure. Changes in cabin air intake design and engine performance gauges cause the pilot to shut down the wrong engine and prepare for an emergency landing at East Midlands Airport which is just across the M1 motorway to Kegworth. On approach to the airport the damaged left engine fails completely and then catches fire. The crew tries to glide the aircraft to the runway but it bounces over the M1 and crashes on the embankment of the motorway, killing 47 people.
Shutting down the wrong engineEditThe Captain, Kevin Hunt, believed the right engine was malfunctioning due to the smell of smoke because in previous Boeing 737 variants bleed air for the air conditioning system was taken from the right engine. However, starting with the Boeing 737–400 variant, Boeing redesigned the system to use bleed air from both engines. Several cabin staff and passengers noticed that the left engine had a stream of unburnt fuel igniting in the jet exhaust, but this information was not passed to the pilots because cabin staff assumed the pilots were aware that the left engine was malfunctioning.
After disabling the Auto Throttler, the left engine flames stopped as they weren't being pumped fuel.
It was a coincidence that the smell of smoke disappeared when the autothrottle was disengaged and the right engine shut down. In the event of a malfunction pilots are trained to check all meters and review all decisions, and Captain Kevin Hunt proceeded to do so. Whilst he was conducting the review, he was interrupted by a transmission from East Midlands Airport informing him he could descend further to 12,000 feet (3,700 m) in preparation for the diverted landing. He did not resume the review after the transmission ended, and instead commenced descent. The vibration indicators were smaller than on the previous versions of the 737 in which the pilots had the majority of their experience.
The dial on the two vibration gauges (one for each engine) were no bigger than a 50 pence coin and the LED needle went around the outside of the dial as opposed to the inside of the dial as in the previous 737 series aircraft. The pilots had received no simulator training on the new model as no simulator for the 737-400 existed in the UK at that time. At the time vibration indicators were known for being unreliable (and in fact normally ignored by pilots) but unknown to the pilots this was one of the first aircraft at the time to have a very accurate vibration readout.
Analysis of the engine from the crash determined that the fan blades (LP Stage 1 compressor) of the uprated CFM56 engine used on the 737-400 were subject to abnormal amounts of vibration when operating at high power settings above 25,000 feet (7,600 m). As it was an upgrade to an existing engine, in-flight testing was not mandatory, and the engine had only been tested in the laboratory. Upon this discovery all 99 Boeing 737-400s (since G-OBME had crashed) were grounded and the engines modified. Following the crash, it is now mandatory to test all newly designed and significantly redesigned turbofan engines under representative flight conditions.
This unnoticed vibration created excessive metal fatigue in the fan blades, and on G-OBME this caused one of the fan blades to break off. This damaged the engine terminally and also upset its delicate balance, causing a reduction in power and an increase in vibration. The autothrottle attempted to compensate for this by increasing the fuel flow to the engine, however the damaged engine was unable to burn all the additional fuel, with much of it igniting in the exhaust flow, creating a large trail of flame behind the engine.