[bookworm]

G-FMKA report only here.

So is the "stall dive" behaviour quoted from the Super Dimona AFM a well known characteristic of TMGs that is not known in SEPs?

[Balliol]

It’s just talking about a stall with wing drop. Due to high aspect ratio of TMGs, incorrectly trying to correct the wing drop with aileron can be more prone to entering the incipient spin than in an ‘slabby’ wing SEP

As a very experienced TMG examiner, I would suggest an FI with no TMG experience would be very ill equipped to deal with a number of the vices a TMG can have.

[allout]

Hi Balliol, me too.
This accident has puzzled us from the start.
Taking a couple of bits from the AAIB report: I'm trying to imagine how one gets a Dimona (any TMG) to 9'g'.
Separately, the report does not detail how they knew who was in command or what the crew's intentions were.
(booking out details, or crew room discussion, perhaps?)

[VRB_20kt]

... a number of the vices a TMG can have.

I'm intrigued by this. Flying TMG well is perhaps a little trickier than say C150, but surely not that different. Even a C150 will drop a wing if provoked. TBH I'm not entirely sure why EASA makes the distinction between SEP and TMG. So what are these vices?

[Dave W]

TBH I'm not entirely sure why EASA makes the distinction between SEP and TMG. So what are these vices?

Whatever they are, they must be wide-ranging since to get a TMG rating from SEP it is deemed essential that the training includes a navigation element. I couldn't see any difference between SEP nav and TMG nav, but no doubt that's because I wasn't looking hard enough.

The actual transition felt more like Differences training to me.

[flybymike]

What were the differences ? (Clearly nav was not one of them; just ludicrous over regulation)

[Dave W]

Adverse yaw; use of airbrake; approach techniques; transition from throttle to airbrake; don't swipe the long wings into other aircraft, bushes, tractors, cars....

Lots of excellent stuff, it's just that the syllabus seemed padded in places.

Interestingly, neither the training or test required the engine to be shutdown/restarted, or prop feathered, in flight.

Also, the expense of getting the Class Rating put on the licence was a pain, and that would not have been an issue if it was Differences Training only..

[VRB_20kt]

My instructor highlighted the wing length in relation to wind gradient when close to the ground. The main thing for me was that my TMG has a tailwheel so there was the learning linked to that to get to grips with (though it doesn't confer tailwheel privileges on an SEP ).

[flybymike]

My instructor highlighted the wing length in relation to wind gradient when close to the ground.

Can you elaborate?

[VRB_20kt]

Friction slows the air nearer the ground so that as you descend, wind speed reduces. It's only really an issue in the 100m or so above the ground. On windy days the effect can be quite marked.

The Grob 109B has a 17m wingspan (compared with ~10m on a spam can) and one of the airfields we practiced at has interesting approaches with a late Base -> Final turn. The consequence is that you're low, slow and turning. If you turn moderately steeply, the lower wing will experience a lower wind speed than the upper one as well as the slightly lower air speed arising from being inside the turn. The effect of this is that lift on the low wing can be significantly less than that on the high wing. in extremis the lower wing will stall and you'll be handling incipient spin at an impossibly low level,

Wikipedia has a decent explanation: "Wind gradient is ... a hazard for aircraft making steep turns near the ground. It is a particular problem for gliders which have a relatively long wingspan, which exposes them to a greater wind speed difference for a given bank angle. The different airspeed experienced by each wing tip can result in an aerodynamic stall on one wing, causing a loss of control accident. The rolling moment generated by the different airflow over each wing can exceed the aileron control authority, causing the glider to continue rolling into a steeper bank angle."

[UV]

This report seems to raise more questions than answers.

They say the drone pics reveal that the wings were bent to such a degrees that would have required a 9G pull. Then it says that the aircraft was seen to be flying normally at 1000 above the ground until just before impact.

To be able to get to 9G the stalling speed would need to be trebled (square root of the G) and would need to be in the region of 3X42 = 126kts. I venture not comparable with witness statements.

To a layman the imprint looks like a vertical impact with a cork screwing motion. I cannot accept anybody was pulling anything like 9G, even the ex RAF pilot.

The report gives very little information about the owner. There seemed a long gap between buying the aircraft and obtaining a SLMG. Was the walking frame his? No age given.

[Balliol]

http://www.nationalprivatepilotslicence ... b%2010.pdf

Has some excellent material covering motorglider air exercises and considerations.

The only nav on the TMG class rating schedule is the normal Section 3 which is on any Class rating test or proficiency check and can be satisfied by a short nav leg to the GH area. The NPPL SLMG test difference is a simulated move off track soaring opportunity and recovery back to planned track.

[Kestrel19]

I would suggest that more common than the speed difference on the wings caused by the
wind gradient on a glider/TMG the far more usual cause of a spin off the final turn is over-ruddering (trying to “help” the aircraft round the turn with the rudder-usually on a shallow turn) and using the elevator to keep the picture looking correct whilst failing to monitor the speed. The aircraft soon ends up out of balance, stick well back, close to the stall - now throw in descending through a wind gradient as well and the aircraft could very quickly decide to stop flying - it won’t matter if it’s a glider, a TMG or SEP aerodynamically they all fly for the same reasons, the spin isn’t caused by the differing airmass but by inappropriate use of the controls.
The above is a commonly demonstrated scenario in gliders whilst teaching spin avoidance, there is considerable emphasis on how misuse of the rudder can be spin inducing.

Please note that I am not suggesting in any way that the above is In relation to the accident in the report!

The one thing that occurred to me with the High G impact however - having (deliberately!) spun a fair number of T-tailed gliders I have yet to find one that on recovery from a spin doesn’t “Bunt” as soon as the rotation stops. In some aircraft this bunt can be very steep and in the more slippery gliders it can easily be possible to be approaching 100knts before the aircraft can be returned to level flight and the height loss in this phase can be several hundred feet more than the rotation phase.
The Dimona has a T-Tail and I could foresee a scenario where this could happen maybe with power on, the speed increases rapidly in the bunt following the recovery and with the stick hard back significant G is pulled but with insufficent height to recover.
Again - I have no idea if this could have happened here but was the only idea in my mind that fitted with a Stall/spin scenario and a 9G pull on impact.

[UV]

Quite agree with the over ruddered final turn Kestrel.

However another scenario,often overlooked, is the overbanked, very steep, final turn. Cable break, field landing etc.
Try spinning a two seater from a steep turn and you’ll be quite surprised how vicious some tame two seater gliders suddenly become. (Including the Ka13).
Add to this a bit of propeller slip stream (in the appropriate direction) on a motor glider and it’s not surprising that it bites you.

The report does say that the aircraft was seen manoeuvring very low with engine noise.

I still don’t see how they managed to get enough speed to pull 9G. (Even recovering from a spin). You would need to be up near VNE (140).

[Nick]

Snipped

I still don’t see how they managed to get enough speed to pull 9G. (Even recovering from a spin). You would need to be up near VNE (140).

A Practice Forced Landing, then full power on and stick right back................and stall? Recover from that.

Nick