Avro York Runway Excursion at Stansted

27 May 22 11 Comments

While researching another subject, I discovered this accident report from 1956 which I just had to share with you. Normally, an aviation accident investigation is done by a group of experts who must come together to create a report of their findings and analyses. A good report is informative and clear in its conclusions; to be honest, a good report is necessarily devoid of personality. This case, however, was investigated and analysed by a single person, who refers to himself as “the investigator” in the final report. There is a clear voice and tone; the author makes a real effort to help us see the environment. The temporary runway was narrow, sure, but not too narrow, and we must surely agree that the “French” drain (always in quotes) is unobjectionable. The bulk of the report is spent ensuring that we do not blame inanimate objects for a crash that no one could have predicted.

The Avro York was a four-engine transport aircraft designed and manufactured in Britain as a cargo-equivalent of the Lancaster heavy bomber. 258 Avro Yorks were produced between 1943 to 1948. There are no surviving Yorks flying today, although it is possible to see one at the Royal Air Force Museum Cosford in England.

The Avro York at Stansted was an Avro 685 York C.1, registration G-AMUL, built in 1946. On the 30th of April 1956, the Air Ministry chartered the Scottish Airlines aircraft for an international non-scheduled passenger flight from Stansted to RAF Habbaniya, Iraq, refuelling at Malta-Luqa Airport.

In 1956, the main runway at Stansted was extended to its present length of 10,000 feet (3,049 metres). During the construction, a temporary runway was created parallel to the runway by extending a 90-foot wide taxiway with shoulders on either side built up of compacted gravel and broken concrete and covered with “close macadam,” which appears to be a more correct term for tarmac.

Stansted Airport in the 1950s; photograph courtesy of StanstedAirport.com

The Avro York had five crew onboard and fifty-one passengers, RAF personnel and their families travelling to Iraq.

During the take-off run, the aircraft suddenly swung violently to the right. The captain closed the throttles to abort the take-off. The aircraft ran off the runway while still travelling 45 knots (52 mph/84 km/h). A drain ran alongside the temporary runway, about 25 feet from the side of the runway. As the aircraft ran over the drain, the undercarriage collapsed and the starboard wheel fell clear. The aircraft skidded on its belly, finally coming to a rest after turning 180°, pointing back at the runway.

When the undercarriage collapsed, the starboard inner propeller cut the control line to the fuel cock and most of the 700 gallons of petrol (3,200 litres) flooded into the cabin. Although the Airport Fire Brigade was able to avert a fire, two passengers were killed (an RAF aircraftsman and a four-year-old girl) and another four were seriously injured.

A scanned copy of the final report is available online on the baa-acro website: Scottish Airlines (Prestwick) Ltd., York aircraft G-AMUL, swung on take-off run and lost a wheel at Stansted Airport.

“Lost a wheel” seems a bit of an odd choice for the title summary when it was actually the port wheel that was the issue, as it blocked the cabin exit.

Due to the belly of the fuselage, as it settled, being forced to starboard, it met the starboard inner propeller which cut the control lines to the fuel cocks with the result that it was impossible to prevent the escape of some 700 gallons of petrol – a large quantity of which poured into the passenger cabin and onto those who were pinned by the entry of the port wheel.

The summary necessarily has relatively few details; what is odd is that most of the above description is never unpacked. The next section, entitled Investigation and Evidence, begins with an analysis of the temporary runway.

The runway is undoubtedly narrow for an aircraft the size of a York, which has a wing span of 102 feet and a wheel base of just under 24 feet. A pilot taking off a York from this runway would naturally seek to avoid letting his wheels go on to a shoulder, although, if he did, it would support the weight as it supported that of G-AMUL on the morning in question, whilst if he hit any of the lamps it would be most unlikely to cause the aircraft the slightest damage. Although admittedly narrow for an aircraft of this size, there is no doubt that this temporary runway is serviceable. The captain of G-AMUL had himself taken off York aircraft from this runway without difficulty on at least 10 to 12 previous occassions and stated in the course of his evidence, when asked whether he was at all troubled on this occasion by the width of the runway, that he was “not concious of any undue narrowness”.

If, as described here, it was instinctive to avoid using the shoulder, the “runway” was just 90 feet wide, compared to 150 feet (46 metres) of the runway which was under construction. It feels a bit mean to ask the pilot if he was aware that it was undoubtedly narrow for the four-engined transport aircraft.

The investigator is also quick to point out that we also shouldn’t blame the York:

The aircraft was properly and efficiently maintained and entirely fit to carry passengers on the flight contemplated.

The York isn’t perfect, he concedes, but that’s not a problem for a competent pilot, apparently:

A York aircraft, in common with many aircraft, has a tendency when rolling to pull to port. This tendency is, of course, well known to all experienced pilots and is not difficult to correct.

Of course.

The engines, the controls and the brakes were all confirmed as having been in working condition before the accident. The undercarriage had fractured at the top, but that wasn’t the undercarriage’s fault either.

Calculations and inspection of the fractures made at the Royal Aircraft Establishment at Farnborough after the accident show that the stresses imposed on the undercarriage when it met the drain were increased to the order of some three or four times those which it was already undergoing and to an extent which no undercarriage is designed to support whilst the fractures disclosed no sign of fatigue but, on the contrary, tensile strength very much above the specified minimum.

The stresses were imposed by the aircraft crossing a “French” drain which ran parallel to the runway. The drain redirected the surface and ground water away from the runway; it was effectively of a deep trench with a cement pipe laid into a concrete bed. The trench was then filled with “rejects”, stones and pieces of concrete, which were piled up high to allow for settlement.

The filling was left “proud” and now that a year has elapsed since the work was done, it is in many places at least 6 inches “proud” while the stones at the top are large and in some cases could be described as small boulders.

It seems obvious that a fully-laden transport aircraft running over an embankment of small boulders is a problem. Our investigator confirms that the aircraft was still swinging to starboard with wheels skidding to port when it crashed into the drain causing the undercarriage to collapse. Ideally, he writes, there should be an expanse of grass on either side of the runway.

Apparently, however, we should not be too quick to blame the drain.

The drain was of a type which appears unobjectionable and in accordance with standard practice. It was properly sited and necessary and the investigator does not think that the fact that some criticism may be made of the manner of filling made any real difference in this particular case.

Other than snapping off the undercarriage which led to the exit being blocked and fuel pouring into the cabin, you mean?

This aircraft left the runway in the most unusual circumstances and it is considered that the blame for what occurred cannot be put upon the drain.

I think we can all agree that the drain did not cause the runway excursion. The strip of “small boulders” running alongside the “undoubtedly narrow runway” was just sitting there minding its own business. However, neither was it the runway excursion that caused the fuel line to be severed; that was the drain.

This confusion between inciting incident and probable cause comes up in modern accident reports as well, to be fair. Two fatalities and four serious injuries were not caused by the runway excursion. They were caused by the collapse of the undercarriage. The undercarriage would not have collapsed if the aircraft hadn’t skidded across the drain. Thus, I can and will apportion some blame, at least, upon the drain.

But the report has dismissed this line of reasoning and returned to the known issue of the York’s pulling to port on the take-off run and a very detailed explanation as to how to deal with it.

There are four throttles (one for each engine) consisting of four levers projecting downwards from the throttle box which is fixed rather above the first pilot’s head and to his right front. It is, of course, to the left but otherwise in a similar relationship to the position of the second pilot. The method of operating the throttles is for the pilot to grasp all four in his hand inserting his fingers between the levers so that he can push these forward to open the throttles or pull back to close them, whilst by an inclination of this hand to one side or the other as he opens the throttles he can advance the port throttles ahead of the starboard or vice versa. In taking-off there is normally no question of closing the throttles and the pilot is occupied in pushing them forward until he attains the desired speed – correcting his course by advancing one pair of throttles beyond the other as may be necessary. Thus, to correct the York’s tendency to roll portwards it is generally necessary to advance the port throttles slightly in front of the starboard. The pilot, once he has got the aircraft rolling straight and at the desired speed, requires his right hand to join his left with which he has been holding the control column and accordingly the practice is for the second pilot to keep his left hand close behind the right hand of the pilot and ready to take over the throttles when the pilot relinquishes them.

This brief tutorial brings us to the actual history of the flight. On that morning, the aircraft was on the threshold straddling the white line but pointing 5° to the left of it. The captain, whose experience as a pilot is never considered, definitely recalls the aircraft pulling to the left.

It moved slightly to the left. I corrected the take off run. The aircraft seemed to come straight. Then I felt a violent pull to the right. I did not like it. I pulled everything [meaning the throttle levers] off and continued on. The aircraft seemed to roll fairly well. After I had got my hands off the throttles I was preparing to use control of the brakes to pull the aircraft up. The aircraft seemed to roll off. The next thing we were off in a 180° turn. Then of course we sat down.

Note the investigator’s choice of the word “insistent” when he rephrases the captain’s actions.

The captain was insistent that he had not at any time used his brakes but thought that he might have used his rudder instinctively. When he closed the throttles he did so because he had decided to abandon the take-off run in view of the swing which had developed. Asked what could have been the cause of the violent swing to starboard he said: – “I cannot think of one myself. The only possible thing I could think of was that I must have somehow over-corrected.” He added that he was “not conscious of having over-corrected” and that at the time he closed the throttles, which was before the aircraft had crossed the centre line, he thought his speed was “fast enough to cause trouble but not too fast to get out of it.”

It strikes me that some leading questions were being asked to get those answers. So did the other pilot think that the captain had caused the issue?

The first officer’s account is that as the aircraft left the threshold he had his head down watching the instruments. He said – “I was aware we were moving to the left of the centre line – it was definitely not a swing in any way but a slight movement to the left of the centre line. The boost pressures were at this time +16 lbs. each approximately. I put my left hand up by the captain’s right hand. I anticipated taking over from him and had my hand I think on the throttles and I felt him using differential throttle and still juggling with them and I looked up to see why. I saw that we were on the left-hand side of the runway – I would not know how much but towards the left. The captain pulled the throttles back and put his right hand on the control column. Almost immediately, I pulled back No. 1 throttle – the port outer – because it was not fully closed.” He went on to describe the increasing severity of the swing and the outcome and stated that the port outer throttle was “not as much as half open” when he himself closed it but more than would have been the case if it had merely bounced back a little on being closed. He was not conscious after putting up his left hand that the captain made any violent or abnormal movement of the throttles.

The No. 1 throttle was not fully closed; however, it doesn’t appear to have been open for long enough to make a substantial difference to the runway excursion. The first officer did not notice any strong correction initially and he states directly that he noticed no violent or abnormal movement of the throttles.

The third quote is from the man himself.

The investigator states that, in the light of the evidence, he cannot think that this swing can have developed without some grave error on the part of the captain.

The violent swing at so early a stage of the aircraft’s run could only result from a correction of the portward course due either to a sudden and excessive differential use of the throttles or to a momentary application of brake to the starboard wheel or to both these factors. The captain was not conscious that he did either of these things.

Neither was the first officer; in fact, he specifically said the opposite: he was not aware of any sudden or excessive differential use of the throttles.

The investigator has no doubt that the captain over-corrected violently and excessively when he used the throttles to bring the aircraft straight and this caused the beginning of the swing.

The captain doesn’t think this is what happened. The first officer doesn’t think this is what happened. Is there any real evidence of this violent and excessive manoeuvre? At the very least, even in 1956, I’d expect the investigator to highlight his suspicions by saying he could not find any other cause.

I should be fair. He does highlight an important piece of evidence.

In light of the starboard wheel mark * and despite the captain’s belief to the contrary, the investigator is inclined to think that he must at the same time have also applied the starboard brake. The latter supposition is necessarily speculative but nothing else in the investigator’s opinion accounts for the sudden development of so severe a swing that even before he crossed the centre line he decided to close all throttles and to abandon the take-off. The fact that in closing the throttles he left the port outer open would, since it had been at 16+ boost, serve to accentuate the swing, but the effect of this error was quickly corrected by the first officer.

The starboard wheel mark sounds somewhat damning, so let’s skip to the end to see what that asterisk brings us.

  • It was observed from the track of his tire that the initial tire mark was that of the starboard wheel and that at this point the width of the track was over 24 feet and accordingly somewhat wider than the normal track width of 23 feet 9 inches. Although the wheels are so set that they can float to a tolerance of some inches, the start of the track and the fact that the starboard wheel track was the first to appear are in the investigator’s opinion important factors.

Not quite the smoking gun I thought it might be. Also, it seems a bit odd to include an important factor as a footnote, rather than part of the findings.

The investigator then questions whether the captain should have aborted the take-off under the circumstances.

It is difficult, without experiencing the violence of the swing as the captain did, to attempt to judge whether his decision to close the throttles and to abandon the take-off was the right decision. Equally, it is not easy to criticize what he did or failed to do after he had closed the throttles. In the latter stages of the wing use of the rudder would hardly have influenced his course whilst experienced pilots who gave evidence expressed the opinion that it was better at this stage not to use the brakes.

Possibly I’m now just being picky: “It is not easy to criticize what he did or failed to do” gives me the feeling that the investigator is disappointed.

In these circumstances the investigator is not prepared to condemn the captain’s decision to close the throttles or his subsequent failure to control the course of the aircraft. The error was committed earlier when he started to correct his portward course.

That would be the inciting error which did not cause the fatalities and injuries in the cabin. But somehow, we never come back to what happened to those people. I found an unsourced quote with seems to have come from contemporary media :

Two die in airliner crash.: An R.A.F. man and a little girl were killed – and four injured when an airliner with 54 people aboard-crashed at Stansted Airport, Essex today. The four-engined York airliner was taking off with families and Servicemen for [Cyprus?] and Irak when it plunged off the runway. Women with babies in their arms scrambled out as patrol gushed from the plane’s shattered tanks. Dense petrol fumes turned the inside of the plane into a gas chamber. Firemen pumped oxygen into the wreckage to keep the trapped people alive.

The scene of the crash at Stansted; editorial use of the photograph licensed from Keystone Press.

Within this context, you would think that some part of the investigation would be on the undercarriage collapse or the cut fuel line or how the passengers ended up trapped. But none of these issues are dealt with.

PROBABLE CAUSE: An over-correction of the portward course of the aircraft possibly accompanied by some application of the starboard brake caused the aircraft to swing starboard off the runway and to encounter the ‘French’ drain with the resulting failure of the undercarriage. The over-correction by the pilot, whether or not accompanied by some application of the starboard brake, should be termed a grave error of judgement and skill rather than a wrongful act or default.

I’m not sure what the difference is between a grave error of judgement and a wrongful act.

This being the 1950s, it’s not surprising that there’s no consideration for cockpit resource management (other than the positioning of the hands over the throttles) or pilot training or the management culture at Scottish Airlines; in fact, the operator is not mentioned at all outside of the title.

My point here, though, is not to argue about whether the captain made a mistake. It isn’t even that the investigator insists that the captain “over-corrected violently and excessively” despite the lack of evidence.

My point is that this a classic case where the investigator is happy to follow the path of least resistance. Blaming a pilot is easy. Blaming the airport or the aircraft manufacturer or the operator is much tougher. If the probable cause can be limited to the grave error committed by the pilot, then the only thing we need to fix is one person. No one has to consider the more complicated issues, for example, the design of the aircraft or the appropriateness of the narrow runway or the ditch covered with small boulders running alongside that temporary runway to keep it dry.

Our investigator does redeem himself somewhat as he finally considers the heart of the matter when it comes to the recommendations. But notice here how carefully he reiterates that the airport did nothing wrong:

Whilst the investigator did not think that the manner in which the “French” drain was filled had any significant effect in the circumstances of the accident, it must be recognized that if the top of the trench is left over “proud” to the extent that it was left in this case and if the stones at the top are of the size employed here, danger could still arise if an aircraft had left the runway in more normal circumstances.

Once he accepts that runway excursions can and do happen, he concedes that something should be done.

There can be no justification for a filling which involves risk to an aircraft if it runs off the runway for a distance as short as 25 feet. It is recommended that, in the case of this particular drain, steps should be taken to reduce the extent to which it is over “proud” to at most two inches and to substitute for the top layer of stones at present in position, smaller stones less likely to cause damage.

At least this. Although, again, he has played it safe by recommending is a relatively easy fix. Meanwhile, at no point in the report does he consider the issue that passengers were trapped in a cabin full of petrol; in fact, there’s no detail at all as to how the injuries and the fatalities occurred. The fatalities were not relevant to the runway excursion which shows just how lopsided the accident investigation was.

Close-up of the foam-covered wreckage of G-AMUL

That said, I’ll concede again that the accident took place on what we might still consider to be early days of aviation and it is, perhaps, unfair to apply such standards to a 1950s accident report.

For a more positive outlook, let us consider this example as a milestone showing how far aviation investigation and safety have progressed over the past 50 years.

Modern accident reports might run to hundreds of pages instead of four and they certainly do not have the character and voice of this one, it’s true, but they do a lot more for aviation safety.

Category: History,

11 Comments

  • Looks like a scanning error – surely “servicemen for Cyprus and Iraq”?

    A wrongful act is an act that is criminal, or at least tortious, i.e. for which someone injured by it could sue the committer. (And a wrongful default is a failure to act, ditto.) An error of judgement, even a grave one, is not an action subject to lawsuit.

    In other words what I get from that passage is “he made a mistake, but it’s a mistake that it would be unreasonable to expect pilots never to make”. Which, taken with the tone of the rest of the report, suggests that the pilot is going to be quietly retired with full blame but not prosecuted, and nobody will have to look any further into potentially embarrassing other issues.

    • Ah, thank you for the help with that. It would be nice to know what happened to that pilot, really. The aircraft is easier to trace: it was a write-off.

      I put Cyprus in brackets in the news quote, nice catch.

  • Well, it sounds like someone got “surprise! you’re an accident investigator now!” and was sent out with no training. Most of what you mention would have been picked up by a trained accident investigator, so I would have to think he had none. Considering how he’s careful to not blame the airport, I would assume he was airport staff.

    1956 is a bit late in the game to be doing this, but it sounds like the entire organization was sloppy, with the poorly done nonstandard runway and everything.

    I thought in WWII at least the military had trained investigators, right?

    I have a firm feeling they hit a divot or hole or other irregularity in the ground that snagged a wheel and caused it to veer off.

    • Ah, that could be. I loved his writing style but he did seem to be in over his head. When I saw the photograph, I couldn’t believe the drain!

  • Were runways checked by the airport authorities for holes, debris or other factors that could cause an accident in 1956?
    It must have been a fairly big pothole to “snag a wheel” to the extent of causing it to break off. More so since the speed was not yet dramatically high.
    Had there been a fatigue crack in (part of) the undercarriage?
    Too much time has elapsed to draw a proper, factual conclusion.
    Aircraft accidents in general do not leave DNA to provide a retrospective “whosedunnit”. In this case there is virtually nothing.
    The investigation seems to have been very superficial, to say it politely.
    There won’t be a file with clues, no “smoking gun”, not even “residual smoke marks” are left behind.
    A tail dragger can be very tricky on the ground (I have 3700 hours on Cubs, Tiger Moths, Stampe SV4, Aeronca as well as a few hours on Curtiss C46 and Beech D18). The Beech was very hard to handle on the ground and actually had a lockable tail wheel to keep it straight until it lifted off the ground. The captain did not even allow me to handle the Curtiss Commando on the ground. It is quite possible that e.g. the DC3 had a similar arrangement. The larger aircraft with tailwheel especially, due to their greater mass and attending momentum, could be a handful.
    My very cautious guess is that the pilot(s?) did not monitor the takeoff properly; asymmetric power: maximum take-off power was not set properly on the port engines. This caused a deviation to the left of the runway, which already was on the narrow side.
    When the crew suddenly realised that they were heading for the left edge of the runway, full power was restored on the port engines and the swing was corrected.
    My very cautious guess is that they acted suddenly, hurriedly and overcorrected. Perhaps the swing to the left was corrected with right braking, all while already starting to accelerate.
    The corrections were no longer properly coordinated, they were a hurried response to a situation where the aircraft was heading to the (left) edge of the runway.
    The C of G of a taildragger is behind the main undercarriage, so a sudden swing causes a momentum that wants to increase the turning tendency: a taildragger acts a bit like a pendulum.
    In the case of this incident, the speed was still low so the effect of the aerodynamics (especially rudder) would not have been sufficient yet to overcome the momentum of the aircraft that caused a swing, the main force caused by the part being behind the wheels.
    Of course, I must stress that this is pure speculation and NOT supported by any factual evidence.

    • I agree that it seems likely and that they acted hurriedly (not violently!) without time to coordinate or even really think about what they were doing. Your speculation seems to be at least as solid as the investigators!

  • it seems the blame for the plane falls mainly on the drain. (my apologies to My Fair Lady.)

  • From a reader:

    I do wonder if they actually pumped ordinary AIR into the plane, which could have been easier and more intuitive. Perhaps they meant “air” when they said “oxygen.”

    I don’t really approve of quitting my job and literally running away as fast as my legs could carry me, but told to “grab this hose, stick it inside the airplane full of passengers and fuel, and I will start pumping oxygen in there” I might have changed my mind about that!

  • So basically,
    • if they hadn’t used the makeshift runway, and
    • if the pilot hadn’t swerved, and
    • if the drain had been filled flat, and
    • if the undercarriage hadn’t detached, and
    • if the fuel line hadn’t ruptured, and
    • if the cabin hadn’t flooded,
    then nobody would’ve died.

    And with a small spark at a bad time, many more.

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