We used HF in Sabah on 230 Sqn. 4469 USB (?) The airwaves would resound with "Butcher Butcher Butcher, this is Baker Baker Baker". You were supposed to pass a position report whenever you left an LZ but you were very unlkely to get through. Quite often there would be heavy breakthrough from activity in Viet Nam. On one occasion some worthy was trying to pass a position report to Butcher and this American voice came up.
"GET OFF THIS GODDAMMNED FREQUENCY! DON'T YOU KNOW THERE IS A WAR ON?"
"Yes; actually I do. But we're winning ours."
For the benefit of India Four Two and the risk of boring a load of ex Whirwind bods to tears here goes.
The aircraft was probably around 10,000lbs, 2,200lbs overweight; but no difference in stresses compared with a tie down base.
The Saga of Stall Margin checks on the Gnome.
Those that fly helicopters powered by the GE T58/de.H/RR Gnome will know that it has a ten stage axial compressor with three stages of variable stator vanes. The angle of these vanes is changed by a fuel pressure powered actuator to suit the RPM of the engine. The faster the engine goes the more the vanes open up to improve compressor performance and as it slow down they are closed up so give the front stages a better bite at the air. There is a RPM point at which the actuator starts working as the revs rise and another point at a higher RPM where it starts working as the revs fall. They found that engines were stalling even though the actuators had closed correctly. This was put down to general corrosion and wear and tear. To over come this it was necessary to ensure that the engine would not stall during and at a certain level below the actuators operating range. This was called the Stall Margin.
It must have been a de Havilland problem because with all my knowledge of T58 operations they do not seem to suffer. It first arose at Tern Hill in the days when places like Sheffield and the Potteries were going full blast and the haze layer went up to 4,000 ft.. To ascertain how badly the compressor was eroded a procedure known as the Stall Margin Check was incorporated.
The aircraft was strapped down to a Tie Down base. The aircraft was started and as the pilot accelerated the engine the RPM at which the actuator commenced its travel was noted. The engine (and Rotor) was then accelerated to about 20,000 RPM and then slowly decelerated until the actuator stared closing again. This was your actuator range check. The Gnome had a built in electric fuel shut off valve tied in with the overspeed protection so this was jumped into a cable of which the other end had a box with a big button. This button would stop the engine; instantaneously. This would have been mounted before the runs started so the engine was shutdown using it as the check.
The engine was started again and accelerated to 20,000 RPM. Whilst the engine was kept at this level the guide vane operating lever would have a locking sleeve attached to it to prevent it being moved by the actuator. So now the engine was running with the vanes locked in the high speed position. The engine was now decelerated slowly; through the two actuator operating RPMs and then below. It would then reach a point where the engine would stall. The Chief Tech in the left hand seat would press his stop button the and pilot would drop the lever and pull off the HP cock as a backup. Should both of them be too slow the engine RPM would disappear left and the PTIT gauge would disappear right. This was not unknown.
IIRC a minimum of 1,400 RPM was required below the RPM at which the actuator started to move. Any thing from Zero to 800 RPM was common.
As a result of doing innumerable checks my confidence in the Gnome overspeed protection was not very high which is probably why it was isolated on the aircraft. I had several occasions where the electrics didn’t work the second time but though practice and knowing what was going on I saved the engine. When the engine went THUMP a very switched on Chief Tech Heatherington could stop the engine before the ‘U’ but other Chief Techs did not seem to fully realise what was going on. The practice of having a unofficial maintenance test pilot helped as he did all the tie downs and air tests. A Joe just pulled out of the crew room would have got away with it most times but if something had gone wrong, as was proven, you lost the engine.
Last edited by Fareastdriver; 8th Aug 2012 at 14:43.