Recreational Aircraft Association of NZ
RAANZ P&P manual
The Trike Tumble- what it is and how to avoid it.
RAANZ May 2010
We have recently had two fatal accidents involving weight-shift controlled microlight trikes. The CAA accident investigation reports are yet to be released and the exact cause and sequence of these accidents may or may not be confirmed in the reports, but CAA believe tumbles are the probable final and fatal mode, and have asked RAANZ to reinforce the understanding and avoidance of a tumble in trike flight training.
The tumble, also known as the tuck and tumble is an unrecoverable mode that all tail-less aircraft are susceptible to. In the early days of tail-less aircraft research the Northrop YB-49 and XP-79, the de Havilland dH-108, and the BKB-1 tail-less glider all had in-flight incidents where the tumble was implicated. Much more recent, the tuck and tumble is well-known to hang-glider pilots- particularly when attempting loops, exiting strong thermals and other extreme flight attitudes.
A trike tumble is an unrecoverable mode, rapidly resulting in separation or breakup of the aircraft structure, generally resulting in severe pilot injury or death. It is the equivalent of a stall/spin on finals for a conventional 3-axis aircraft- unforgiving and frequently fatal
Trike pilots should understand what a tumble is, the conditions of entry, and how to avoid them. It should be included in our training syllabus, and reviewed as part of any flight test briefing.
Mechanics of a tumble
From the airframe's perspective, the rate of rotation is often of the order of 360°/second- the control bar and front strut and wing leading edge generally fail within only a few rotations of the tumble.
Some hang-glider pilots have recovered from a tumble by throwing a chute, but the rapid rotation and physical beating up by the control bar make it extremely unlikely that even a prepared trike pilot would be able to deploy a ballistic chute under these conditions.
The only sure way out of a tumble is to not get into one.
Causes of a tumble
The entry to a tumble is from a steeply nose up attitude with low airspeed. There are 4 documented modes of entry:
Avoiding a tumble
The rules are fairly simple
And if you find yourself in those entry conditions
The tumble in trike flight instruction
There is no safe air exercise to take the aircraft into tuck/tumble entry and recover. We recommend a briefing with diagrams, models and viewing a video of a tuck/tumble. It is recommended that discussion of the causes of a tumble and correct recovery be part of each check flight to ensure continued pilot awareness.
So are trikes safe?
No worse than any other aircraft. All forms of aviation- gyro-copters, powered parachutes, helicopters and 3 axis fixed wings have a safe operating envelope, and operation outside of it can lead to an unrecoverable mode. The key of course is for each pilot to be aware of his aircraft's safe operating flight envelope and ultimately his responsibility to always operate within these limits.
In the case of a weight-shift trike, this is the one unrecoverable mode
Resources and further reading
Dr Guy Gratton of the BMAA has researched and written the definitive papers on this issue.
The FAA Weight-shift Control Aircraft Flying Handbook page 2-19 describes a tuck/tumble, Page 6-24 onwards covers tumble avoidance.
There is art least one video of trike tumble from a failed loop on the internet- sobering viewing, but a useful training tool to embed in the pilot's mind 'don't even try it'.