Tiger Moths with leading edge slots - a salutary tale.
 

Browsing the Pprune non-airline forum Accidents and Close Calls, I came across an incident involving a Tiger Moth at Mackay in July 1951. The elevators had been incorrectly rigged during maintenance. The pilot was a TAA DC3 first officer who had flown Typhoons in the Royal Air Force shortly after the end of WW2 and who was test flying the Tiger Moth prior to delivering the aircraft to Melbourne.
The take off on the test flight was dramatic as described in the pilot's report. He wrote:

"On take-off the machine waddled down the runway then leapt into the air of its own accord. I was suddenly aware of climbing with 30 knots on the clock, the auto slats standing open like the legendry clutching claws of fate and the aircrafts nose still rising despite the fact the stick was full forward. Not a pretty picture.

To gain airspeed the machine was stood on its port wingtips; the nose dropped and problem number one was solved. Problem number two soon emerged - the aircraft insisted on a tight left hand turn which couldn't be controlled with rudder; not really disturbing as a turn was necessary for landing. However, the left hand turn took us straight towards about six high frequency (HF) radio masts complete with aerials and guy wires.

Reduced bank produced a hop over that obstacle and a slipping descending turn was made to line up with the runway. Beaut! Except for problem number three which became evident as the wings were levelled for landing, the nose popped up and we were climbing again.

The second circuit was like the first although speed was reduced to allow the wings to be levelled for landing ; the reduction was insufficient and once again we were climbing. Third time around proved lucky, the machine made quite a respectable landing by stalling completely as the wings came level.

Had anyone been interested, three circuits with the stick full forward in less than two minutes would have been some sort of record.

So what went wrong? The control box was incorrectly assembled and at some stage the down-elevator cables became slack. Back stick gave up-elevator correctly. As the stick was moved forward, slackness in the down-elevator cables allowed the elevators to fall under their own weight. On take-off as the stick was moved forward to raise the tail the elevators took up a streamlined position behind the tailplane and airspeed increased.

Unknown to me was the fact that in straight and level flight the elevators are depressed by about 15 to 20 degrees. With the elevators streamlined behind the tailplane, a strong nose-up pitch force is experienced by the aircraft. Hence the aircraft left the ground in a tail-down position.

Fortunately, I had received good training with the RAAF and RAF on Wirraways, Miles Masters, Spitfires and Typhoons in authorised low flying, stalls, spins, aerobatics and recovery from unusual attitudes. All proved valuable".
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Several de Havilland designed aircraft including the Tiger Moth, came equipped with leading edge auto-slats (known as slots) that came open at an angle of attack approaching the stall. They made a distinctive clacking noise as they opened or closed and were a remarkably effective stall warning device. They were operated by a lever in the cockpit and for taxying and aerobatics were locked to prevent their damage. Pre-take off and pre-landing drills included the requirement to unlock the slats to lower the stall speed.

It was the automatic operation of the leading edge slats that would have prevented the Tiger Moth in question from stalling at full power when it leapt into the air of its own accord at 30 knots.

Tiger Moths currently in service with private operators around Australia have their slats permanently moved; ostensibly to reduce maintenance costs. Yet over past years there have been several stall accidents in Tiger Moths - some fatal - whose auto slats had been removed by maintenance organisations. Thus loss of stall warning. False economy, maybe?:ugh:

Don't think so Centaurus, think of Austers, Chipmunks and the like, no stall warning and they seem to fare OK.

My first 70 hours of power flying were in a Tiger Moth with the slats locked closed. One of those hours included a balloon bursting competition where I managed to get very close to stalling in tight turns (at a safe height and with an instructor on board). The handling was quite predictable and safe.

That is nice to know. One interesting statistic is that the de Havilland DH 82 Tiger Moth first flew in 1931. It was operated by the Royal Air Force, the Royal Canadian Air Force, and in many other countries, becoming one of the best known primary trainers of World War II. During the early years of the war, the aircraft was vital to the British Commonwealth Air Training Plan as thousands of Commonwealth pilots' first solos were in Tiger Moths. A total of 8389 Tiger Moths were built by de Havilland and its licensees between 1931 and 1945.

Just think the millions of dollars that could have been saved and put towards the war effort if all these 8389 Tiger Moths had been put into service without their slots since they are now, in hindsight, considered unnecessary in terms of flight safety:ok:

A million dollars then would have probably built all 8389 from scratch:)

Tiger Moth automatic slats worked very well and saved many lives including mine when ham fisted students turned steeply onto finals that could, on occasions induce a flick stall, Megan's comments about Chipmunks and Austers not having stall warning devices fitted is true but these aircraft fared well because they had flaps which the Tiger did not. When slats were wired up the accident rate went up. It was not so much because of costs but more to do with poor Pilot procedures when not locking slats when carrying out aerobatics etc.
Slats were a lifesaver in their day and still are on many modern jets.

I well remember the locked check and unlocking of the slat lever on the Tiger prior to landing.
And the advisory clack ! at the required speed on touchdown...if you got it right.

Was the removal in surviving Tigers of the operating cable and slats due to that wing failure accident in WA...some 20 ?? years ago.??
Decades of vibrating cable in the spar hole, along with a split. had weakened the top outer wing structure so it failed on pull out of a loop. 2 fatal.

Auster J5P once owned, had a stall warning horn...and its sudden loud blare made me jump, first time I heard it !
Auster J 2...No flaps ...and no stall warning. Back to basics.

Discussion here. Opinion seems to be why did DH ever fit them, seems they were an option on private purchases.

Tiger Moth auto-slats [Archive] - PPRuNe Forums

I’ve flown a couple of Tiger Moths with slats and three without. Four of them had a tail skid and no brakes, one of them had a tail wheel and brakes. One had hand holds built into the wingtips, the others didn’t, some had different exhausts, I think one had the straps on the oil tank and the others didn’t. Whatever. They were all different in one way or another. I found the slats to be of no great value and preferred to have them locked all the time, mainly so I wouldn’t forget to lock them prior to aeros. I found the Tiger Moth to be incredibly benign in all flight regimes. That said, I don’t dispute they may be useful in extreme edge of the envelope cases or with brand new ham-fisted students, but I always liked to stay away from the envelope’s edges and never had the pleasure of instructing on anything, let alone a Tiger Moth.

All in all, I don’t find a story about a freak misrigging incident to be adequate reason to suggest that all Tigers should have operational slats.