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?
