Tumble

The Trike Tumble- what it is and how to avoid it.

RAANZ May 2010

Introduction

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

Attach:tumble.jpg
1.A tumble is normally entered from an extreme nose high attitude with minimal airspeed.

2.The wing will either stall and rapidly rotate nose down, or may enter a tail-slide where the trailing edge billows upward and again causes the wing to rapidly rotate nose down.

3.By itself, the wing pitch stability due to reflex may normally cause the wing to resume flying and recover from the ensuing dive.

4.But if the pilot aggressively pulls the control bar in to reduce the nose high attitude, the rate of downward rotation may be accelerated to the point the wing enters a negative angle of attack and pitches nose down beyond vertical (the tuck)

5.Alternatively, if the pilot rapidly reduces power, the trike base may rotate until the front compression strut hits the control bar and again forces the wing to rotate rapidly nose down beyond vertical (the tuck).

6.As the wing noses down past vertical with negative angle of attack, the control bar forces the pilot and trike base to rotate backward up and over the top of the wing (now inverted).

7.The trike base continues its backwards rotation over the wing and around towards normal attitude, at the same time the front strut forces the control bar and wing to continue a rapid nose-down rotation as in step 5.

8.And around we go again (the tumble)

From the pilot's perspective, the control bar thrashes rapidly backwards during step 6 and rapidly forwards during step 7. The bar forces are far greater than the pilot can overcome, and ribs are often broken.

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:

The whip-stall. Caused by an aggressive entry to a stall from high speed and/or under power, with an aggressive push out of the control bar to put the aircraft in a very nose high attitude. Often followed by an aggressive recovery action by the pilot puling the bar in.
Spiral instability with loss of horizon. Trikes have an inherent spiral instability- when put into a turn they will tend to increase the roll angle and may enter a subsequent spiral dive unless 'top bank pressure' is applied. For pilot caught in IMC conditions and initiating a turn to get out, this can result in a high speed dive followed by a panic zoom and power-off recovery from an extreme nose high attitude.
Failed loop maneuver. Trikes are not approved for aerobatics or negative G maneuvers Vertical or inverted with no airspeed on the clock and the pendulum weight you need to control the wing on the wrong side...enough said.
Flight through own (or other) wake vortex (or other severe turbulence). Trikes and other delta winged aircraft generate a significantly larger wake vortex than 3-axis aircraft of similar weight. The BMAA recommend that steep turns be limited to 270� without changing altitude to avoid encountering your own wake. Given that weight-shift attitude control depends on a positive G environment with pendulum weight below the aircraft centre of gravity, any turbulence that removes these conditions can result in an uncontrollable attitude.

Avoiding a tumble

The rules are fairly simple

Fly within the safe envelope.
Avoid the above known entry conditions.

And if you find yourself in those entry conditions

Hold the bar out. Do not pull the bar in. This is to avoid a pilot initiated rapid pitch down.
Apply full power. Do not reduce power. This is to help prevent a pitch down of the trike base and subsequent forced rotation of the wing.
Apply roll input. Convert the attitude from a steep pitch to a steep bank, then reduce power and fly out of the ensuing spiral dive.

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.
A plain English analysis and explanation of tumbling. Much of this article has been drawn from this paper.
A detailed technical analysis of the tumble mode for those interested in reading further.

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'.