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Aviation History
1957
1957 - 0129.PDF
1 February 1957 •m- Dr. Touch's Report on the Vulcan Accident ON Thursday of last week the second report* on the Vulcanaccident at London Airport on October 1 last year waspublished. It dealt with the investigation of the accident by Dr. A. G. Touch, Director of Electronics, Research andDevelopment (Ground), M.o.S.—an investigation requested on October 26 by the Minister of Transport and Civil Aviation.Dr. Touch's terms of reference were "to consider whether [at the time of the accident] there was any failure to operate correctlythe [G.C.A.] at London Airport and, if so, to consider in what respects and whether, in the light of the circumstances of theaccident, any changes are desirable . . ." He conducted his in- quiry informally as a private investigation under Accident Regu-lations, and was not able to use evidence given at the R.A.F. Court of Inquiry. "An approach under bad weather conditions," says Dr. Touch inhis introductory paragraphs, "is a difficult flight operation; in my opinion, far more difficult than is normally recognized. Whatever aid is beingused, whatever 'coupler' is employed in the aircraft to interpret the information gleaned and translate it into aircraft motion, the processcan be likened to a servo-loop. . . . "... the only certain fact we know is that the loop as a whole failed.It will be seen later on that the real issue in this accident is which portion or portions contributed, and some of the main evidence sug-gesting that the G.C.A. rectangle was at fault concerns the improbability of the aircraft having carried out the flight path as given by the G.C.A.record . . ." After remarking that "R/T. communications throughout were verygood," Dr. Touch goes on to say: "The Vulcan first touched the ground 1,988ft from the runway threshold, 250ft north of the extended centreline, while still under G.C.A. assistance. At impact the aircraft was in a normal approach attitude, with nose up. The rate of descent was notexcessive and probably was of the same order as for a normal landing, viz., about 180ft/min. The airspeed was 140 to 145 knots . . . "The official weather conditions . . . were: wind calm, heavy rain,visibility 1,100 yd, scattered low cloud down to 300ft (2/8), and 7/8th cloud at 700fL However, conditions were patchy. At the timeof the accident there was mist and slight drizzle in the region of the runway threshold. Horizontal visibility looking outwards along thecentre line was about 800 yd ... "Further back along the approach path, at approximately 1J n.m.from touch-down, horizontal visibility at ground level was around 200 yd, but the aircraft was seen at a distance of about 400 yd. In thisarea it does not appear to have been raining heavily . . ." Answering the question "Did the G.C.A. equipment function pro-perly?" Dr. Touch describes examinations made immediately after the accident and says: "It can be concluded with confidence that errors insetting up the displays, incorrect calibrations, or malfunctioning of any part of the equipment did not occur." Talk-down Evidence Examined He then goes on to examine the possibility of "unusual effects" such asradar "ghost" echoes due to the Vulcan's characteristic plan-form, but, in general, dismisses these considerations. The next section of his reportreviews in considerable detail the evidence provided by the G.C.A. record, eye-witnesses, the pilot and co-pilot, the controller and thetracker (a woman air traffic control assistant with exoerience of some 2,000 G.C.A. approaches). After examining and dismissing variousexplanations suggested by consideration of this mass of material, Dr. Touch makes this statement: "The simpler solution is that the trackerfollowed the aircraft echo more or less accurately, but that the con- troller did not pay sufficient attention to the error-meter after the point'80ft. high.' ^'It can be seen from the transcript at the talk-down that after thispoint, the controller's patter was concerned with range and corrections in azimuth. This shows that he was mainly looking at the azimuthdisplay. It is generally recognized that it is difficult to watch both error-meter and display at the same time. "On this basis, it is possible to formulate a theory which appears tobe a reasonable compromise with the evidence. After '100ft. low' (100530)f to 'on the glide-path' (100541J) the rate of descent appearsto have been low, of the order of 150ft/min, and this was noticed by eye-witnesses. Afterwards to 100551 it appears to have increased to atleast 300ft/min. It is therefore possible that the tracker may have erred slightly on the hieh side in bisecting the echo. If the latter limit for thetime of impact, 100601, is taken, the tolerances given . . . [here an earlier reference is quoted] ... are possible, and the resulting rate of descent,although high, is acceptable. During the descent, it is not unreasonable to postulate that the tracker lagged behind by 50ft, especially if a fairlylarge echo was being used. The error-meter would have reached zero about 5 to 6 sec prior to impact and the warning light would have comeon about 2 sec before impact. If the tracker switched her light off when she lost the echo just after half a mile, it means postulating that thecontroller did not observe the meter for a period of 7 to 8 sec. This is not unreasonable, as he was busy with azimuthal corrections and hislast two elevation messages had been spaced 10 sec apart. If he glanced at the meter during the first 5 sec he would be left with the impressionthat the aircraft was returning to the glide-path. The warning light came on too late to be of any value. . . ." * Ministry of Transport and Civil Aviation, C.A.P. 143—"Report ofthe Special Investigation by Dr. A. G. Touch into Certain Aspects of the Accident to the R.A.F. Vulcan Aircraft B.I XA897 at LondonAirport on 1st October, 1956. H.M.S.O., price 3s 6d. •fThis is an expression of time, i.e., 10 hr OS win 30 sec.—Ed. In the next section of his report, headed "Discussion," the investigatormakes, inter alia, the following observations: — "I am unable to postulate a really satisfactory theory that would placethe aircraft at a lower altitude than that given by the controller near the finish of the approach, to allow for a less steep descent prior to impact.But if any such explanation could be given, it follows that the aircraft must have lost that height elsewhere during the approach . . . "I therefore accept the G.C.A. record as being correct, subject to thenormal tracking, observational and timing errors. In elevation, it is typical of a servo-mechanism out of control. It follows that I acceptthe existence of a steep dive after the point '80ft high,' since it provides the only rational explanation ... I have been able to find . . ."The aircraft crossed the centre line roughly at 4J miles and was always north of it, in spite of three 5-deg corrections. As a result thecontroller gave much more attention to azimuth than is normal, and this left him less time for giving elevation and range information. "Although the controller did not consider that he was under stress, atleast he was being hurried more than he thought. A small sample of talk-downs by other controllers chosen at random show a spread in wordsspoken per minute of 82 to 126, with an average of 110. On this occa- sion, the controller averaged 146, obviously reacting to the increasedground speed . . . "Legally, the pilot was entitled to receive enough height informationby which to fly his aircraft safely and on which to base his decisions. This he did not get. On the other hand, if the flight-path previous tothe '80ft high' point is not taken into account, the time interval of the critical phase is so short that no one could blame the controller for notpassing further height data. The issue is, therefore, was the last steep descent unexpected, or could it have been foreseen from the previousflight path so clearly that the controller should have concentrated upon the elevation situation? I find this question exceedingly difficult toanswer, but on balance ... I think the verdict should be that the circum- stances were such that it should be classed as a true accident and thatthe controller was not to blame. "I must conclude that the talk-down in elevation was poor and,because of its negative character, contributed towards creating the final critical stage. The controller is only responsible for passing instruc-tions and information; he is not in control of the aircraft. The ultimate responsibility for the safety of the aircraft and for the decision whetherit is safe to descend below the break-off height, rests with the pilot. Consequently, even if the talk-down was poor, it was subject to theoverriding judgment of the pilot, and the controller cannot be blamed for subsequent events arising from the poor control . . ." Dr. Touch's conclusions and recommendations are as follows. Wequote them almost in full: — Conclusions "(1) The G.C.A. equipment was correctly set up and calibrated, there is no evidence of malfunctioning or failure. "(2) The controller failed to warn the pilot of his closeness to the ground. "(3) During the last 10 sec of the approach the aircraft made a steep descent to ground. "(4) The cause for this descent was probably due to the build-up ofoscillations about the glide-path. Poor 'talk-down' by the controller con- tributed to this, but as the approach was subject to the overriding judg-ment of the pilot, the controller was not to blame for events arising from the control. "(5) The critical phase was the first 4 sec after the descent steepened,during which no height guidance was given to the pilot ... in view of all the circumstances, I do not think the controller should be blamed. "(6) No warning was given during the final 5 or 6 sec. It should havebeen, although it would have been too late. Although it cannot be definitely proved, the most likely theory is that the controller made anerror of judgment, concentrating too much upon azimuthal corrections, and paying insufficient attention to the elevation error meter. "(7) Human errors are more likely to occur under stress ... Inmy opinion, evidence exists to show that all the elements in the G.C.A. servo-chain were strained." Recommendations "(1) . . . military aircraft of advanced design should not normally beallowed to land at civil airports under bad weather conditions, unless the break-off height is very conservative. "(2) The record of G.C.A. at London Airport is very good, and thesystem as at present used does not require any radical change. As aircraft speeds increase, however, the controllers will have less time inwhich to pass instructions. Unnecessary words, such as 'you are . . .' 'just,' 'quite' and repetitions in the patter should be avoided . . . "(3) Visual guidance in azimuth and range is adequate almost as soonas contact is made with the approach lighting pattern, but is not adequate in elevation until quite a low altitude is reached. Height data fromG.C.A. should therefore be given to as low a height as possible. Danger of undershoots can be reduced, possibly at the expense of a slight increasein overshoots, by concentrating on elevation data during the last stages of the approach. Serious consideration should be given to a pro-cedure in which elevation data only is provided after the pilot has gone visual, or after the break-off height. "(4) Split responsibilities should be avoided, if possible. A G.C.A.system in which the controller observes both sets of displays, elimin- ating the need for a tracker, is to be preferred ... "(5) If G.C.A. is to continue as a primary approach aid (as distinctfrom a monitor) it is recommended that photographic recording of the displays should be carried out in addition to recording the talk-down ..,"
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