Biggin Hill Accident Report

1 Oct 10 One Comment

On the 30th of March 2008 a Cessna Citation 500 took off from Biggin Hill. Shortly after take-off, one of the pilots reported engine vibration and declared their intent to return to Biggin Hill. They never made it to the airfield. The Citation crashed into a house and the two pilots and three passengers died on impact.

In May 2010, the Air Accidents Investigation Branch released a formal report on the accident. It makes for chilling reading.

The flight was cleared to take off from Runway 21 at 13:32. The first transmission after take-off was their intent to return to the airfield.

13:34:25 (1,560 feet amsl):
AND VICTOR PAP BRAVO GOLF ECHO ER WE’RE MAKING AN IMMEDIATE TURN TO RETURN TO THE AIRPORT IMMEDIATE TURN TO THE AIRPORT

13:34:32:
VICTOR GOLF ECHO JOIN DOWNWIND RIGHT HAND RUNWAY TWO ONE THE BIGGIN HILL QNH ONE THOUSAND THRESHOLD ELEVATION’S FIVE ONE SEVEN FEET WHAT’S THE NATURE OF YOUR PROBLEM

13:34:39 (1,540 feet amsl):
ER WE DON’T KNOW SIR WE’RE GETTING ER ENGINE VIBRATION WE’LL COME STRAIGHT BACK

The AAIB were unable to find any issues with the engines that could cause such a vibration. The report concludes that the vibrations the pilots reported felt were not caused by an engine at all. The air cycle machine (ACM), part of the Citation’s pressurisation system, was the likely culprit.

The damage observed on the ACM fan bearing cage confirmed that it had operated after failure of the cage. Operation in this condition would result in the relative position of the rollers around the shaft changing, allowing increased lateral movement and vibration of the shaft and the fan attached to it.
[…]
It follows that, in the event that the shaft was vibrating, any change of thrust on either engine would result in a change in the characteristics of the vibration being produced. These shafts run at high speed so any vibration would be similar to an engine vibration frequency. The investigation concluded that vibration of the ACM shaft and inlet fan was the most probable source of vibration that the pilots described as “ENGINE VIBRATION”.

At this point, the pilots are likely to be attempting to work out which engine is causing the vibration. The instruments are no help because both engines were actually fine. However, reducing the power to either engine would have changed the vibration which, the AAIB state, could have allowed the pilots to misidentify the engine as being the source of the vibration.

Having “identified” the engine with the problem, the pilots could well have decided to shut it down.

13:34:43:
VICTOR GOLF ECHO THAT IS UNDERSTOOD THE CIRCUIT IS CLEAR REPORT ON FINAL YOU ARE NUMBER ONE AND ANY RUNWAY AVAILABLE IF REQUIRED

13:34:50 (1,690 feet amsl):
ERRRR WE’LL COME STRAIGHT ROUND ON TWO ONE SIR WE’LL COME STRAIGHT IN.

13:34:58:
VICTOR GOLF ECHO IF YOU COULD JUST ADVISE US TOTAL ON BOARD PLEASE IF YOU COULD.

13:35:03 (1,700 feet amsl):
ER WE HAVE FIVE POB SIR WE’RE COMING STRAIGHT BACK ROUND WE’LL JOIN ERM ** LEFT HAND COMING STRAIGHT ROUND FOR TWO ONE IF THAT’S OKAY.

13:35:12:
VICTOR GOLF ECHO AFFIRM ANY ROUTEING IS FINE AND ER RUNWAY ER IN FACT JUST CONTINUE THE APPROACH THERE’S ONE VACATING SURFACE WIND AT THE MOMENT IS TWO FOUR ZERO DEGREES AT SEVEN KNOTS.

The plane was flying at half power, consistent with one engine being shut down. However, the vibration would have continued.

At this point the flight begins losing height. Based on the AAIB simulation, “the total thrust is calculated to be less than zero which would mean that neither engine was running.”

We can certainly see that the aircraft is losing height faster than one would expect.

13:35:20 (1,440 feet amsl):
THAT’S ALL COPIED THANK YOU VERY MUCH.

The investigation found that there was a missing rivet on the left fuel cut-off lever.

It is conceivable that, with the missing rivet on the left fuel cut-off lever, the action of selecting idle thrust with the left thrust lever may have led to the left thrust lever inadvertently being placed in the fuel shut-off position. If the right engine had previously been shut down as a result of the vibration, the aircraft would then have been left with neither engine running.

The recorded speed of the plane was 130 knots.

13:35:29:
VICTOR GOLF ECHO RUNWAY TWO ONE YOU’RE CLEAR TO LAND TWO FOUR ZERO DEGREES AT SEVEN KNOTS

13:35:34 (1,400 feet amsl):
THAT’S COPIED WE’RE JUST COMING ROUND THIS TIME.

At this point, they need the thrust of one engine, either engine, in order to pull the plane out of the descent.

The relight drills in the manufacturer’s checklist call for the boost pumps to be turned on; the engineering investigation identified that the left boost pump was running at impact. The checklist also calls for the airspeed to be increased to 200 kt, if altitude allows. At the point identified when neither engine was running, the aircraft was at 130 kt and 850 ft agl. Simulator tests revealed that it was not possible to accelerate to 200 kt from 145 kt in the height available (the best glide speed). From 130 kt, this was clearly not possible.

The analysis of the wreckage led the AAIB to the conclusion that the first engine had not completed its start sequence before an attempt was made to start the second engine.

13:35:47:
SURFACE WIND NO NEED TO ACKNOWLEDGE IS TWO FOUR ZERO DEGREES AT SEVEN KNOTS

[13:35:52: OPEN TRANSMISSION FOR THREE SECOND – NO MODULATION – SOURCE UNKNOWN]

The manufacturer’s checklist also states that only one engine should be started at a time. One engine was all they needed but both engines were operating at the point of impact. Trying to relight the second engine before the first engine had reached idle speed appears to have lost them their final chance to prevent the crash.

A sense of urgency due to the proximity of the ground or confusion over the problems they were dealing with might equally have led to a deliberate attempt to start the second engine before the first engine had reached idle speed. The effect of doing this would have been to delay the start of both engines. It is probable that, although both engines were operating at impact, they were in the process of accelerating to their demanded output and unable to provide sufficient thrust for the aircraft to climb away.

13:36:43 (45-145 feet above crash site elevation):
AND ER VICTOR GOLF ECHO WE HAVE A MAJOR POWER PROBLEM A MAJOR POWER PROBLEM IT LOOKS AS THOUGH WE’RE ER GOING IN WE’RE GOING IN

Sulako posted an accident summary based on the report as well as an interesting post with his analysis.

Sulako’s Blog

My take on it is that once they were convinced that it was the right engine that was sick (the left engine would have still been producing enough thrust to keep them climbing, so maybe that’s how they decided it was the right engine), and when pulling the thrust lever back to idle didn’t stop the vibration (again, the vibration was the air cycle machine, which had nothing to do with the engines, and it would have continued as long as either engine was operating), they decided to shut it down completely.

So far, I can see how that would happen.

The vibration continues after the right engine is shut down, the crew goes “unfortunately it appears we shut down the wrong engine” or words to that effect, and they decide to go to plan B.

The big question is: why were the pilots so insistent on shutting down the engines. If they had simply turned around and returned to Biggin Hill without doing anything else, it would have been a trivial incident. Having shut down an engine and discovered the pitch had changed but the vibration hadn’t stopped, they had a further chance to ignore the vibration and concentrate on reaching the airfield.

One commenter on Sulako’s blog asks if perhaps the vibrations were so severe, the pilots thought structural damage might occur if they allowed it to continue. That makes sense to me, the juddering aircraft filling both pilots with the overriding desire to do something.

In this sad case, doing nothing would have been the best solution of all.


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Category: Accident Reports,

One Comment

  • The 200 kts for an inflight re-light applies to an air-assisted (windmilling) re-start.
    Using the starting motor, the engine (“classic” Ce500) can be re-lit at ANY speed.

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