CLOSE TO DISASTER – A RISKY HELICOPTER FLIGHT TEST

There is always the question when you tell people you are a test pilot of ‘Isn’t that risky?’ and invariably most test pilots answer by saying ‘not really’.  The problem with both the question and the answer here is that invariably the former ignores the fact that there is some degree of risk attached to all aviation activity and the latter ignores the fact that in the test world we do an awful lot of work to reduce risk to a minimum, but never completely eliminate it.  The nature of flight test is , by its very nature, to find out what the limits of system are and that doesn’t matter whether we are talking about speed, the number of targets a radar can track or at the most mundane end of the spectrum whether the new USB port attached the the aircraft actually works.  It’s why the European Aviation Safety Agency introduced categories of flight test in 2015 defined limitations about what qualifications allowed a pilot or flight test engineering to conduct them.

In 2002 we were in a very busy period of the Royal Navy Merlin Mk1 flight test programme and had deployed the aircraft to the Southern Californian desert to conduct Hot weather performance testing; Temperatures were up to 45 Deg C during the day at our test base, US Naval Air Station El Centro.  The trouble with that sort of heat is it generates a lot of low level air turbulence and wind so invariably when we needed to deliver accurate test points we had to do it at dawn, which in August was around 0500 or ‘oh my god its early’.  Our testing involved tying the aircraft to the ground via a long wire strop attached to the helicopters load lifting sling and effectively trying to lift the earth with the helicopter so we could find the real engine and gearbox limitations.

Whilst Agusta Westland (now Leonardo Helicopters) had done some earlier testing of the aircraft’s performance they had not done so in this environment.  We were actually doing this for the first time which at times I think we tended to forget; this was full on experimental flight test, something not to be approached lightly.  In our planning we had certainly tried to capture all the issues especially since in 1992 the Empire Test Pilots’ School had lost a Sea King helicopter doing exactly this sort of testing (but during training).

The aircraft was fitted with quite a lot of instrumentation including a load cell on the cargo hook and we had a full rear seat crew who were to guide us as we attempted to keep the aircraft directly vertical above the tether point; no mean feat when you cannot even see the wire.  For those long line pilots out there we don’t have bubble windows or cameras or mirrors to help us out in an aircraft the size of a Merlin.  In addition we also needed to try and keep the power as steady as possible so the Flight Test team could capture stable and reliable data.  Needless to say we didn’t quite get there immediately and there were a few early morning flights where I was bluntly informed my hovering wasn’t good enough!

Pride still partially intact we set off on the morning in question and went through the same procedure as before this time hoping the wind and my flying weren’t going to let us down.  We attached the wire and lifted up to about 100ft where the wire became taut, got our visual references and started to gather data.  Suddenly from the back came the call, ‘down down’ and we descended quite quickly to make the wire slack, the testing was over. So what had happened?

Well, what the rear crew had noticed was that at one of the 4 points where the cargo hook strop was attached to the aircraft with a turnbuckle, it had bent back on itself, through some fluke of geometry as we had lifted into the hover.  It was the point furthest from the cargo door on the opposite side of the aircraft so actually very difficult to see for someone hanging out of the cargo door whilst trying to keep the aircraft vertically over the wire.  If the turnbuckle had broken with us pulling maximum power the aircraft would have immediately rotated about the new centre of rotation and we would have almost certainly got into an irrecoverable position with only the ground to stop us moving, such is the nature of momentum.

From a procedural perspective we had got too used to the manoeuvre and not paused long enough with the wire slack for the rear seat crew to check that the strop was correctly handing by the four attachment points.  This was in spite of the fact that we understood this was a risky test and that some of our predecessors had been less than successful doing a similar thing.  Perhaps I was too focussed on hovering better or we were all focussed on getting the required data in a limited duration test period.  In any case tethered hovering was no longer possible on this aircraft and we moved on to more mundane things like measuring the cockpit temperature in the middle of the day or how hot the tail skin got when flying sideways with exhaust gases for the engine flowing onto it.

So yes, all flying is risky, but in flight test some things are definitely more risky than others. Sometimes, like when tethered hovering, you know it ,but other times it creeps up on you and catches you out – but that’s another story.