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Aviation History
1981
1981 - 2384.PDF
216 FLIGHT International. 2S July 1981 The Sumburgh report summarised ACRASH to a non-scheduled turbo prop transport in the remote Shet land Islands killing 15 oil-rig workers is only front-page news when, as in this case, there was the human story of a .Dan-Air stewardess's bravery. What should now make this accident an air- safety classic is the report of the UK Accidents Investigation Branch.* The AIB has set the air-safety world another valuable example of accident investigation, analysis and reporting. There are no villains—although hind sight, as always, points blame's own guilty finger. Those who seek an obvious cause and scapegoat will be disap pointed; but those concerned with flight safety in nearly all departments— design, flight-operations, technical sup port, incident-reporting, airworthiness, engineering, airport and cabin surviv ability—will find this report worth read ing. The report "strongly recommends" redesign of the 748's gust-lock system. This recommendation is for the Civil Aviation Authority to implement. The authority has approved the maker's pro posal for a warning-light to indicate Above G-&EKF is pulled from the sea after the rescue (Scotpix photo). Below Side elevations show the gust-lock lever in the accident position (left) and correctly set (right) Plaie rivetted 3^5^ over origi na 1 s&r^ one piece gate 5y^ plate ** FWO /• ' / \m , I m ' / w Lever between detents. telescopic section depressed to stowed position ie lever in false detent Original one piece \ type \5/0 4120 1 gate plate Locks off Gate -slop/ / 1 \ strip k / 0YH FWD \ I KM I i'/ ' Lever between retained in the unstowed position by interference between gate- stop strip and mid section of gate c^ plate. "O *-ocks o*1 f> ?$s. ^.Gatesaal Kc^' x:—£p|*te / «ii#V *—— Removable ' ^s_^ gate plate when the cockpit gust-lock lever is not fully in its forward "controls-unlocked" position. The following is an edited summary of the report's detailed investigation of the gust-lock system. To prevent damage to the control surfaces from gusty winds on the ground, each control system has an in ternal lock. The three locks are operated by a telescopic lever on the right-hand side of the cockpit control console. Operation of this lever permits the controls to be locked during park ing and taxiing, and to be released be fore take-off. The photographs show the relative positions of the gust-lock lever, throttles, high-pressure fuel cocks, gust-lock and flight fine pitch stop (FFPS) withdrawal lever. The gust-lock lever's full back ward movement causes all three control locks to engage; full forward movement causes them to disengage. The upper section of the lever is mounted telescopically in the lower sec tion and may be slid manually upwards against two springs. This vertical up ward movement is necessary before the lever can be moved away from the •Aircraft Accident Report 1/81. Accidents Investi gation Branch, Department of Trade. Her Majesty's Stationery Office, London WC1V 6HB, £5-40 net. locked or unlocked detents. The upper section of the lever has attached to it a rectangular-section strip, termed the gate-stop strip, which engages with one of two detents (known as the "on" and "off" detents) in the console at the "controls locked" and "controls unlocked" ends of the lever's range. Vertical upward movement of the upper section of the lever then with draws the gate-stop strip from the de tent, allowing the lever to move to the opposite end of the range, where the telescopic section may be lowered again, once the gate-stop strip is coincident with that detent. At any position be tween the two detents, the upper sec tion of the lever is prevented from moving to the "down" position by an interference between the gate-stop strip and a gate-plate. Thus to engage or disengage the con trol locks, the lever is lifted fully up wards, moved to the other end of the range and allowed to move fully down wards. If it remains in the "up" posi tion, then the fully locked or fully un locked position has not been reached. This is a design safeguard to indicate to the pilots that the gust-lock lever is incorrectly positioned. Freedom of the controls is normally- assured by the crew shortly before take off by the following actions: 1 insuring that the gust-lock lever is in the "controls-unlocked" (forward) position with the telescopic section "down," i.e. in the "off" detent. 2 Carrying out "full-and-free" checks on rudder, elevator and aileron controls. During the take-off run it is normal practice for the handling pilot to hold the control column fully forward to keep the nosewheel in ground contact until VE. Therefore, during take-off, it is not normally obvious to the crew, from control feel and response, whether or not the elevator is free until back pressure is applied. An interlock between the FFPS withdrawal lever and the gust-lock lever prevents the former from moving forward into the "stops engaged" de tent when the gust-lock lever is in the "controls locked" position. Movement of the throttle to the take-off position auto matically moves the FFPS lever forward to the "stops engaged" position. When the FFPS lever is prevented from mov ing to "stops engaged" by the gust-lock lever interlock, forward throttle move ment is also limited by a mechanism which permits only taxiing throttle. The design intention of this throttle- baulk was that the interlock system should be the primary safeguard against take-off with the locks engaged. Wreckage examination Examination of the wreckage revealed that it was possible to move the tele scopic gust-lock lever to the fully "down" position regardless of the fore and aft position. Once "down," consider able force was needed to move the lever fore and aft. Examination of the gate-plate indi cated that a steel plate had been riveted over the basic metal components. In addition, a steel strip was riveted along a side of the slot. The strip was free to deflect sideways and therefore did not contribute to the lateral location of the gust-lock lever. It has not been possible to determine when this freedom of movement first came into being. Exami nation of these steel riveted components indicated that they did not conform to the drawings for any of the standards of gate-plate produced by the aircraft manufacturer. The distance between the "locked" and "unlocked" detents was found to be approximately 11 per cent shorter than the minimum specified by the manufac turer although, at the time of the acci dent, the dimensions were not men tioned in the technical publications available to operators. Tests established that take-off throttle could be obtained on both when the gust-lock lever was anywhere between the "off" detent and approximately 45 per cent of travel towards the "on" detent. Serviceable HS.748s were used for tests on the gust-lock and associated interlock systems. The first test was to evaluate the theory that under certain circumstances it might be possible for the elevator lock to become re-engaged during the "full-and-free" control checks before take-off. The gust-lock lever was moved to approximately 30 per cent away from the unlocked detent. With the lever
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