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Aviation History
1972
1972 - 3234.PDF
FLIGHT International, 7 December 1972 807 Trident inquiry Tests and procedures As THE PUBLIC INQUIRY into the loss of Trident G-ARPI at Staines on June 18, 1972, entered its second k week the emphasis was laid on an examination of the operating and training standards of BEA, with particular reference to the number of previous incidents with features common to those of the accident. The evidence of the eye-witnesses was heard without challenge by counsel and the tribunal then turned to substantiating the flight-recorder data and establishing the basic aerodynamic parameters upon which the Hawker Siddeley analogue rig reconstruction of the accident had been based. Mr Peter Waller of BEA described how the flight-recorder data had been reduced from its raw state (the form in which it was published in Flight last week on page 770) to a series of corrected graphs which were before the court. His evidence was important in establish ing the validity of flight-recorder read-outs since the court was to hear frequent references to not only the G-ARPI traces but also to recordings made during normal line operation and during certain flight tests. It was under standable that learned counsel might not readily appre ciate the nuances of position error and instrument error, and some time was therefore spent on relating the recorded figures to the likely indications which the pilot would have seen. Although at this stage it appears unlikely that any particular airspeed reading will be needed with extreme accuracy, the exercise helped to show that the indicated airspeed readings to which Capt Key had been flying might not have been quite so seriously below the optimum as had earlier been inferred. Mr Waller explained that airspeed calibration varied depending on whether the speed was increasing or decreasing and said that, allowing 3kt error as "a reasonable probability," the speed at auto pilot engagement could have been 17312kt. When Mr Donald Dykins, deputy chief aerodynamicist of Hawker Siddeley, took the stand the hitherto clear under standing of the subtleties of airspeed indication rapidly dissolved, for his concern was not with IAS or calibrated recorder speeds but with the equivalent airspeed, this being the factor which governed the computer calculations needed to establish the aerodynamic forces on the aircraft and hence its likely behaviour. Once the discussion reached the effect of angle of incidence on indicated airspeed and how the computer would apply the necessary corrections it was understandable that counsel should remark: "I will not ask you anything about the next page [of your evidence] because it is too complicated for me." Referring to a graph of incidence against a time base for the accident flight, Mr Dykins pointed out how the stick-shaker and stick-pusher boundaries had been super imposed, making due allowance for the change of datum which would have occurred as the droops travelled inboard. In the droop-down configuration the shaker operates at 14-25° and with the droop up at 8-6°. "As long as the droop was down, as it was until after second 114, is there any evidence that the stick-shaker would have operated or in fact did operate?" asked Mr Anthony Lloyd for the Attorney General. "This evidence shows," replied Mr Dykins, "that it was very, very close to the tolerances we know of as to incidence and speed. If they all went together then it would be operated momentarily and go off again." Referring again to the corrected graphs, Mr Dykins later went on to explain to the tribunal that throughout the period after the droop began to retract and while the stick-pusher was operating the aircraft was never aero- dynamically stalled. It reached an incidence of 12-6° while the leading edges were still moving and this is the incidence at which the pusher would operate with the droop fully retracted. This incidence was not subsequently exceeded before the stick-pusher was dumped. "It is quite clear from this picture that the aeroplane as an aero dynamic vehicle is very much a flying machine . . . and very much would respond to any movement of the controls. ... It is not a brick that is falling out of the sky." The tribunal having previously heard that the airspeed of PI was below the target VXA of 177kt, Mr Ronald Ashford of the Civil Aviation Authority was called to discuss an analysis of other Trident flights which he had carried out. Taking a sample of 100 flights, the speed on average after flap retraction was "very close to VSA and a speed of 9kt below VKA occurs on about one flight in ten." Mr Ashford agreed with the suggestion that there had been a trend over the years and that a representative figure now would be about one in 30 flights. Mr Ashford said that the study of 100 flights included an investigation of the speed loss on manual and autopilot-controlled climbs and attempted to establish the effect (if any) of a turn during flap retraction. "The results showed an average speed loss of about five knots after flap retraction . . . with no significant effect from a turn; speed losses of up to 23kt were recorded . . . and a loss of 13kt (as in the accident flight) occurs on about one flight in 13 (one in 22 for manual climbs and one in 11 under autopilot control)." The witness could not determine whether speed lock was engaged during the assessed climbs. Turning to his analysis of the test climbs flown for the CAA in Trident G-ARPN, the witness said that these had followed closely the profile of the accident flight and, having selected autopilot at a low airspeed, the test pilot had not "dialled up" to the correct figure. While on the accident flight the speed at engagement was 168kt, lowering to 166kt at flap retraction and finally reaching its minimum figure at 153kt, engagement on the test flight at 171kt left the speed still at 171kt at flap retraction and a minimum of 164kt was recorded. A second test climb followed a closely comparable pattern. Mr Ashford's con clusion was that the loss of speed after flap retraction on the accident flight was greater than normal, though not exceptional, and the very large speed error resulted from this loss and the low speed at flap retraction. Before the start of the proceedings on November 27 the Attorney General took the opportunity to introduce the written evidence relating to the wreckage, which included reference to the discovery of a seat-back folding table upon which abusive scribblings were legible. The evidence of a hand-writing specialist was read in which, having studied the handwriting in the crew members' log books, he made a cautious suggestion of the authorship of certain of the lines. This submission created an impression that one of the junior crew members had been responsible; the record was set straight the following morning, and later in the week evidence was produced which showed that not only were all the scribbles on that particular table 12 days prior to the accident but also that Capt Key had seen similar abusive graffiti elsewhere and had not reacted. Taken in conjunction it appeared that earlier suggestions that the graffiti might have led to discord on the flight deck were not tenable. The opening witness for BEA was Capt Douglas Evans, a Trident flight manager and Trident member of the carrier's operating standards committee. He introduced in evidence volume one of the Trident Operations Manual, itself comprising four volumes, and told the tribunal of the other ways in which operating information was passed to crews. Discussing the composition of crews on the Trident, Capt Evans said that BEA had tried to guard against the use of two inexperienced crew members together by means of the "brown-line" system. This had been introduced in 1965 and meant that a newly qualified second officer would have his name underlined when rosters were being drawn up; this used to apply for one year and the ere wing could take account of inexperience simply by ensuring that no two "brown-line" pilots flew in the same crew. Capt Evans described a particular problem which had arisen at the time that Second Officer Keighley was being trained. It was normally the practice of the corporation to carry out training to the standard needed for a newly
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