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AAIB investigation to Hawker Hunter T7 G-BXFI 22 August 2015

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AAIB investigation to Hawker Hunter T7 G-BXFI 22 August 2015

Old 16th Mar 2017, 09:59
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Hebog,

It is the anti-collision light, a red rotating beacon (standard fit on a T7).
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Old 16th Mar 2017, 11:00
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Thank you Lomcevak - couldn't see it on anything else on the video.


If his G indicator wasn't working correctly, could he have exceeded the aircraft limits without knowing it causing damage either internally or external. As it would appear that the fatigue meter was only checked once a year and not after each flight, as per the AAN. And with the extensive damage to the aircraft some damage may not have been obvious to the AAIB, like the pitot which they stated couldn't be checked for serviceability.
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Old 16th Mar 2017, 12:10
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The display manoeuvres were calculated to be in the region of 3G and well within the capabilities of the aircraft. The non functioning G meter should have read up to 9G, so while damage might occur below this figure, the manufacturers clearly thought that the airframe would survive this amount of force and be capable of landing safely for repairs. I seriously doubt that any pilot would want to pull more than around 5G in what is essentially fun flying for the purposes of entertainment.
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Old 16th Mar 2017, 12:26
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If his G indicator wasn't working correctly, could he have exceeded the aircraft limits without knowing it causing damage either internally or external.
Not at the quoted range of IAS. The limit is 7g, and you need around 420kt IAS to achieve that. The Hunter is a tough bird - there have been intances of 9G+ without damage.
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Old 16th Mar 2017, 12:34
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The Hunter is a tough bird
Is or was? From the AAIB report maintenance was maybe(?) not up to the standard as when she was in service. Not trying to point blame just considering that caveat.
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Old 16th Mar 2017, 14:32
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He was certainly going some on the dashcam clip on you tube. Maybe as a result of the increased RPM that is detailed in the report, which was not usual compared to other video clips of previous flights by the pilot.


Thought that aeros would generate more than 3G, and an earlier display was 5.5G and we have no idea if the aircraft had exceeded limits prior to Shoreham, causing an issue that went undetected. As there is no data on the fatigue meter readings taken at the last annual inspection and during the investigation.


Another AAIB report G-HHUN makes interesting reading with regard to engine surges and stagnation that can occur. Could this explain the unusual behaviour of the RPM detailed in the report for BXFI.
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Old 16th Mar 2017, 15:16
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The conclusion with G-HHUN was that the engine was destroyed by the inadvertant operation of the HP fuel pump isolation switch (when the pilot was reaching for the "smoke on" switch). It's burned into my memory because I was at DUnsfold on the day, helping with preparations for the the next day's Families Day. Whilst the report includes some commentary about potential issues with the bleed valve control unit I'm fairly sure I remember the conclusion was that this would have no discernible effect on the power delivery unless it progressed to a full surge (compressor stall).

There was nothing to indicate a surge or similar event in the case of the SHoreham aircraft was there?
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Old 16th Mar 2017, 16:19
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From the report, the only instance I can find of the word 'surge' is in para 1.6.9

This restricted the fuel flow to the fuel nozzles when the gun was fired,
to prevent compressor surges. This system was deactivated when the cannon
was removed from the aircraft during its military service.
So the system was "deactivated" rather than removed?
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Old 16th Mar 2017, 17:12
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Lemain. The Hunter had the 'Gun Dip' system fitted to alleviate the engine surge problems associated with the disrupted air intake flow during gun firing. This system has indeed been deactivated on all of the civilian Hunters.

The Hunter also had a means to isolate the twin HP fuel pumps to maintain a fuel flow in the event of HP pump failure. This was activated by a guarded switch in the cockpit behind the throttle. This High Pressure Pump Isolation Switch (HPPIS) could result in a considerable over-fuelling of the engine, the consequences of which would be an almost instant over temp/destruction of the turbine. It was concluded that the engine failure of G-HHUN was caused by the pilot inadvertently opperating the HPPIS when trying to activate the aircraft's smoke system.

It was yet another Hunter crash which killed it's very, very inexperienced Hunter pilot.
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Old 16th Mar 2017, 17:26
  #550 (permalink)  
 
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Originally Posted by Lemain
From the report, the only instance I can find of the word 'surge' is in para 1.6.9
I just looked up the bulletin which discusses surge in three contexts - the Gundip system, the BVCU and the chance of stagnation/surge if the engine was mishandled at low speed and high AoA. But none of these make it into the recommendations because they seem reasonably satisfied that the engine was destroyed through HP fuel pump isolation without closing the throttle.

So the system was "deactivated" rather than removed?
According to the above bulletin the Gundip system was disabled under mod KT.1321, but the implementation only disconnected the airframe side of the system. This sort approach was quite common because it negated the need to re-analyse the weight&balance calcs for the ODM and rebalance electrical and cooling systems (complicating the mod and massively increasing the cost). The bulletin also suggests that after an aeroplane was lost due to suspected electriocal noise pick-up in the retained wiring a further mod removed all the wiring, but this was after the aeroplane had been sold on so it wasn't an incorporated mod.

But again, nothing in the Shorham report that I've seen would point towards problems related to the gundip wiring as a potential cause.
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Old 16th Mar 2017, 17:45
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BVCU and system can cause lack of power as i've already posted .You can have correct rpm and jpt but low thrust if bleeds open. I suspect it was discounted as the power was adequate for takeoff which i would have thought proved things with a relatively short runway , tailwind and warm day.
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Old 16th Mar 2017, 18:13
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The Hunter also had a means to isolate the twin HP fuel pumps to maintain a fuel flow in the event of HP pump failure. This was activated by a guarded switch in the cockpit behind the throttle. This High Pressure Pump Isolation Switch (HPPIS) could result in a considerable over-fuelling of the engine, the consequences of which would be an almost instant over temp/destruction of the turbine.
In the interests of accuracy, this system was fitted to the 100-series Avon (and thus relevant to G-HHUN and the Shoreham T7) but was not on the 200-series as used on the F6 and its derivatives.
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Old 16th Mar 2017, 18:58
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Originally Posted by bvcu
BVCU and system can cause lack of power as i've already posted .You can have correct rpm and jpt but low thrust if bleeds open. I suspect it was discounted as the power was adequate for takeoff which i would have thought proved things with a relatively short runway , tailwind and warm day.
While that might be evidence that it was functioning correctly at takeoff, might it have failed in some way during the manoeuvre? If so, is it likely there would have been evidence from the wreckage? Might a failure have conceivably been a contributory factor? I'm wondering if something distracted the pilot?
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Old 16th Mar 2017, 22:53
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Originally Posted by bvcu
BVCU and system can cause lack of power as i've already posted .You can have correct rpm and jpt but low thrust if bleeds open. I suspect it was discounted as the power was adequate for takeoff which i would have thought proved things with a relatively short runway , tailwind and warm day.
Re the above, if the bleed valves are open, that will produce a higher JPT for a given rpm. Thrust (mass flow rate) is directly related to the compressor delivery pressure and the fuel flow (given fixed compressor and turbine geometry). Open compressor bleeds reduce the compressor delivery pressure (CDP) for any given selected throttle position, this results in the accel control unit scheduling an increase in fuel flow, through the servo fuel system to produce a CDP increase to restore the mass flow rate. An engine producing a given thrust with bleeds open will have a higher rpm and jet than the same engine at the same thrust with the bleeds closed. I've seen this on testbeds on the Avon & Spey, and a number of other engines
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Old 17th Mar 2017, 00:21
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on the aircraft the marginally higher JPT on a 100 series was not visible on this type of gauge. In this day and age you would probably see it on a digital gauge but not on those. This was the reason we were doing daily engine runs to check bleed valve settings in the early eighties when there was an issue with the BVCU diagphram material until they were modified. That was checked on strip according to the report so can't see it would have been the problem. We have come a long way in terms of engine instrumentation nowadays ! If there had been a noticeable lack of thrust the RPM would have been continuously at the top end of the scale to achieve enough thrust you would think , so the thrust reduction doesn't make sense unless there was a transient high RPM/JPT fault which caused him to momentarily throttle back ? We'll never know.
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Old 17th Mar 2017, 00:34
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so the thrust reduction doesn't make sense unless there was a transient high RPM/JPT fault which caused him to momentarily throttle back ? We'll never know.
I'm concerned why the AAIB report didn't cover that possibility?
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Old 17th Mar 2017, 15:32
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There is data in the report regarding RPM, albeit from an analysis of the action camera audio, and JPT as viewed on the cockpit video data. The relationship between the two was consistent and, therefore, there appears to be nothing to suggest a bleed valve problem. This is probably why it was not analysed or investigated further. With a 100 series Avon the normal manifestation to the pilot of a bleed valve malfunction is a low JPT for a given RPM when at a high value. The pilot does not have a thrust meter and any differences in acceleration due to a variation in thrust are subtle and may not be noticed other than by an acceleration check on take-off. The only guaranteed indication to the pilot of an Avon 122 with a bleed valve stuck open is a JPT of less than 560 C at maximum RPM.

For those who seem to be intent upon trying to identify an engine malfunction I would like you to consider the fact that the aircraft reached the apex with sufficient airspeed to enable an escape manoeuvre to be flown safely even with idle thrust which is not significantly greater than the zero thrust from a flamed-out engine. And it would be an uncanny co-incidence for an engine malfunction that caused a significant reduction in thrust to occur during a manoeuvre which the pilot entered at an airspeed less than the minimum planned and during which he also failed to perform an escape manoeuvre despite being significantly below the gate height.
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Old 17th Mar 2017, 16:30
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For that number of holes to line up, there would need to be no cheese at all.
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Old 17th Mar 2017, 16:57
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Originally Posted by LOMCEVAK
And it would be an uncanny co-incidence for an engine malfunction that caused a significant reduction in thrust to occur during a manoeuvre which the pilot entered at an airspeed less than the minimum planned and during which he also failed to perform an escape manoeuvre despite being significantly below the gate height.
Can you explain your logic ?

The less-than-full-thrust, low entry speed, low apex height and failure to perform an escape manoeuvre that you cite are all AFAIK undisputed facts.

So why does attributing the reduced thrust to an engine malfunction turn this into an "uncanny coincidence" ?
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Old 17th Mar 2017, 17:15
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DaveReidUK,

In hindsight I should have said 'very unfortunate co-incidence' rather than 'uncanny'. The reduced RPM and JPT are due either to a pilot selection or an engine malfunction. An engine malfunction is a relatively low probability event and so it would be very unfortunate if it coincided with a manoeuvre when the required parameters were not achieved and the correct course of action was not followed.
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