Wonwings diary-Tiger Moth spin entry & recovery.
To:Air Accident Investigation Branch,
Farnborough House,
Berkshire Copse Road,
Aldershot,
GU11 2HH.
01-08-2006
G-ACDJ Tiger Moth,crash 18-08-2005
Dear Sir,
I am writing to you to offer some information that could possibly be of some use to you in your investigation of the crash of the above aircraft,having read in detail the excellent report I feel that something important has been overlooked,I will add that this is in no way intended as a criticism merely to highlight and possibly add to your findings,I hold nothing but respect for the AAIB and the difficult and demanding work that it does in the interests of furthering air safety.
During the sixties I was deeply involved in the maintenance and repair of De Havilland DH.82A Tiger Moth aircraft,sometimes even going on post restoration test flights that entailed the spinning of these machines,the characteristics varied very much from one machine to another,some aircraft were reluctant to spin,whilst others went into the spin very quickly,there were examples whereby the aircraft was reluctant to pull out of a spin one of which resulted in a rather lengthy court case between the previous owner.
Whilst the fitting of anti-spin strakes would assist the pull out of a spinning Tiger Moth this was not always the case,it is well known that the the blanketing effect of the rudder was the result of slipstream around the rear fuselage without the strakes,the history of the strakes was the direct result of the wartime fitting of bomb racks underneath the wings of Tiger Moths which caused instability problems laterally and caused many Tiger Moths to crash under then unknown circumstances.
So how stable is a Tiger Moth laterally ? well this is a complex subject not helped by the history of some much restored aircraft,it is well known that G-ACDJ a particularly vintage aircraft had been repaired many times over the years and is probably a far different aircraft than it first emerged at Hatfield back in the thirties as a brand new aircraft,subsequent repairs can add a lot of extra weight to any aircraft and notwithstanding the aircrafts normal weight and balance schedule,something else must be remembered,lateral stabilty is very much allied to lateral static balance something that is often overlooked,in other words the aircraft should balance in the lateral axis,just the same as it does in the fore and aft sense,well how exactly is this checked as I have never seen it mentioned anywhere in any aircraft repair manual ? well the only way is to suspend the aircraft from the engine crankshaft via a simple jig that will lift the nose from the ground,the rudder is then locked and a simple gimbal joint placed underneath the tail skid spoon so in effect the aircraft is suspended between the two points,with two people on either wingtip the aircraft will start to roll one way,industrial weights or quantities of ballistic plastercine is then placed on the light wingtip until the wings become level,this will determine whether the aircraft is statically out of balance.
It will be necessary to ensure that the hangar floor is level by checking with a spirit level before conducting the above test,but it does work.
Most Tiger Moths will when throttled back start to turn under the torque of the propeller effect,any further bank could be attributed to either warps in the aircrafts wings or any inbalance as described above,depending on the severity of same will determine how unstable the aircraft could get,and at a worst case scenerio start to enter an insippiant spin.
The case with the bomb racks as fitted on some wartime aircraft was a warning how outloading underneath the lower wings would upset the balance of such aircraft.
The hidden dangers are wings both top and bottom that are of unequal balance due to numerous repair work that has been undertaken that can add an alarming amount of weight.
I submit these suggestions in the spirit of continuing aircraft safety and from personal experience in maintaining and operating these delightful aircraft,and above all trust that you will find my notes of use in the future.
Thank you and I look forward to your comments.
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And the reply received on 10th August 2006 from Alan.P.Simmons,Principal Inspector of Air Accidents-
DearSir,,
Thank you for your interesting and informative letter dated 1st August 2006,I have discussed the contents with the inspectors involved with this accident,and can advise you of the following points.
We did check the aircraft’s weight and longitudinal balance,and although this was not detailed in the report,it was within the correct range.We were unable to check the aircrafts lateral balance,and this is not recorded in the documentation.We did check the aircraft’s trim settings,and there was no evidence of any unusual trim position.The investigators also spoke with other pilots who had flown this aircraft,and there were no reports of any unusual behaviour.
We are aware of issues with spin entry and recovery with Tiger Moths and examined a number of flight test reports before forming our conclusions.
In short we do not think lateral balance was an issue,but if it had been we would have no way of reliably proving it.Nonetheless I thank you for your thought-provoking comments.
