Here I deal with some of the more significant accidents related to Comet Mks. 1, 1A & 2.
[Those relating to the Mk.4 are dealt with under their respective airline entry or here].
The Comet had an excellent safety record and was much respected by the industry as a whole. The incidents detailed below should not be miss-interpreted as meaning that the Comet had more than its share of ill luck … that was definitely not the case.
Such was the importance of the Comet that minor ‘incidents’ became particularly significant.
Detailed below are incidents that had a major influence, one way or another, on aircraft design, construction, operation or on safety; on public opinion or, in a general sense, on industry practice world-wide.
Two accidents in particular stand out: The loss of Yoke Peter and Yoke Yoke caused a major rethink on a subject, hitherto little understood, that of metal fatigue in highly stressed structures. These tragedies effectively killed the Comet 1 and almost killed the ‘Comet’.
Canadian Pacific Airlines
In the Canadian Pacific Airlines incident ground ‘stall’ occurred on 3rd March 1953. CF-CUN ‘Empress of Hawaii’ crashed on take-off at Karachi on it’s delivery flight to Sydney, Australia – killing eleven. ‘Hawaii ‘ had commenced its take-off run and, before reaching V1, it appeared, the pilot ‘rotated’ [lifted the nose] too soon to generate enough lift – ground-stalling.
The 1A was at the weight limit for the conditions. The crew had planned to leave at night so that the air temperature would be lower but, even so, it was still 8° above ISA (International Standard Atmosphere). Water/ Methanol injection was used to compensate for the Ghost’s power loss. There was no wind.
No official report was published into this accident. But a summary was issued later which stated, inter alia, “at this high weight, strict compliance with the take-off technique would be necessary for a successful take-off”.
Again The International Federation of Airline Pilots Associations were concerned about the summary conclusion which amounted to ‘pilot error’ by Capt. C. H. Pentland.
The Association were frustrated in their investigations and felt that not all the relevant evidence had been made available to them. They wanted more information as to the actual stalling speed of the Mk.1A, in the prevailing conditions. Weight, ambient air temperature and humidity were taken into account, particularly in the light of subsequent knowledge as to the adverse influence of ‘ground effect’ on the stalling speed. They reasoned – it was possible that the Karachi Comet was “scheduled to unstick at one or two knots above or below the stall”.
Concerning the accident to Yoke Zulu the International Association again argued: if there had not been a problem with the Comet 1’s ground stall characteristics why was there a new paragraph dealing with take-off in the proposed UK edition of the International Standard of Recommended Practices?
This edition advised that ‘no attempt should be made to take-off until a speed of 1.15 times minimum ‘unstick’ speed is reached. These ‘margins’ may be reduced to 1.1 or 1.05, relatively, when the limitation is due to undercarriage geometry, and not to ground stall, characteristics’.
All the same the loss of CF-CUM was a mystery. But, as stated previously, the two accidents had one thing in common – both aircraft were taking off when relatively ‘heavy’ and at, relatively, high ambient temperatures. In these unique circumstances, on rotation, the wing stalled.
Of course this was the point. At normal weights and relative temperatures the Comet could take-off even with the tail bumper hard on the ground – a maneuvers de Havilland Test Pilots demonstrated regularly.
Test Pilot Peter Bois was involved in the training of CPA crews and confirms they had the ‘technique’ demonstrated to them during both day and night take-offs.
Peter Bois theory
Peter Bois has another theory for the accidents – that extreme fatigue was a major contributor to the Canadian loss. Not often mentioned in official reports was how long the pilots had been on duty at the time of the crash. To summarize: because of scheduling and extended flight times the Canadian crew would have been on duty for some 40 hours – probably without sufficient sleep.
The tragic fact was the Canadian crew almost got away with it. Score marks indicated that the Comet was airborne for a short time and then struck a low stone wall bordering a deep gully.
On 25th June 1953 another Mk.1A was written off at Dakar, Senegal when F-BGSC, belonging to Union Aéromaritime de Transport (UAT), skidded off the runway. No one was hurt but the aircraft, which only been in service for just over seven weeks, was an insurance write-off!
So there were a series of incidents all related to Comet stall characteristics. Later Comets were to be fitted with drooped leading edges – with which de Havilland had been experimenting – with the intention of permitting higher all-up-weights.
There was criticism too about the power assisted controls – they were said to lack feel. In general though pilots resisted any suggestion of change – the systems simplicity and lightness outweighed any risk of over-control. de Havilland continued to work on this though and the result was the introduction of ‘Q feel’ – first seen on the development Mk.3. As the name suggests, the controls, although still fully powered, had been given an artificial degree of feel and were much more pleasant to use.
G-ALYV – Yoke Victor
This was a serious loss. On 2nd May Yoke Victor broke up in a violent tropical storm near Calcutta.
Newspaper headlines ran ‘Airliner knocked down by tempest’. There had been reports of extreme turbulence at approximately 10,000 ft in cumulous-nimbus clouds. Opinion was that the storm would have downed any aircraft.
37 passengers and a crew of six were lost – commanded by Capt. Maurice William Haddon – one of BOAC’s original Comet Captains and a very experienced pilot.
The last contact with the aeroplane was 6 minutes into the flight when the pilot routinely reported that he was climbing – the aircraft would have been at approximately 10,000ft.
Comets had flown over a 100 million revenue passenger miles and had carried 28,000 passengers, but there was much concern because of the apparent suddenness of the accident. A special meeting was convened in London with senior executives headed by the BOAC Chairman Sir Miles Thomas. After a careful review of reports they decided that, for the time being, scheduled Comet services would continue.
British crash investigators assisted an Indian Investigation team. T. R. Nelson wasSenior Inspector of Accidents, Ministry of Civil Aviation Accident Investigation Branch and J. B. Folliott was BOAC’s Chief Investigating officer.
The British authorities came to the conclusion that the loss of the aeroplane was directly or indirectly due to the severe turbulence. However the Indian Court of Inquiry decided the ‘probable’ cause of the loss was due to,
“sever gusts encountered in the thunder-squall, or over-controlling or loss of control by the pilot when flying through the thunderstorm.”
Either way the results of the Inquiry were unsatisfactory. BOAC and de Havilland issued a combined statement which made it clear that they did not agree that over-control or loss of control was the likely cause. They concluded that the Indian findings were theoretical and were not based on a detailed examination of the wreckage.
Peter Bois Theory on Yoke Victor
It is important to point out that the true reason for the loss of Yoke Victor was never established. Very little wreckage was recovered as it was distributed over a wide area. The truth will never be known but it seems likely that structural failure did occur – but why?
Peter Bois, a de Havilland Test Pilot during those years, proposed one personal theory to account for the loss of control and subsequent break-up of the aircraft.
Explaining that the Captain of Yoke Victor was extremely experienced and highly respected by his colleagues. They considered it impossible that he would have taken the aircraft into extreme turbulence at speeds higher than those recommended. Conversely, could he have taken the aircraft into the turbulence at too low a speed? It was possible. He may have concluded that such a manoeuvre may be the safer option. Certainly he may not have realized that it was perhaps far more dangerous to fly into severe turbulence at below the recommended rough-air speed. He was not after all a test pilot.
Peter Bois describes the strange sensation that can be experienced. Reducing speed,
“can easily lead to a low speed stall. The stall is followed by the aircraft pitching nose down and accelerating very rapidly to a speed vastly beyond the aircraft’s structural limit.”
Peter explains that unless one has experienced it oneself it is very difficult to appreciated the degree of, “lateral upset” which can be induced.
On one occasion, this was a DC8, extreme clear-air turbulence was encountered which resulted in the aircraft being thrown into a sixty degree bank, “against full opposite aileron.”
The flight recorder registered structural stresses close to structural limits – the crew thought they were going to lose control at one point!
“this took place under visual flight conditions, in daylight. The Comet accident took place at night and in cloud, so it seems possible that the aircraft became inverted. There was also some evidence in the wreckage, consistent with this idea.”
The Elba Crash – Yoke Peter
While the cause of the Calcutta accident had not been determined there was still considerable confidence in the Comet within the Corporation, de Havilland and with fare-paying passengers. The Comet was breaking records regularly and passengers appreciated the advantage of reduced journey times. There was nothing to match the quietness and smoothness of the pure-jet.
By early 1952 BOAC Comets had accumulated about 30,000 hours operating over some 12 million miles. Along with Air France, UAT and the Canadian Air Force the Corporations Comets were flying some 180,000 miles a week!
Against these statistics the Calcutta accident began to look like a one off.
Then disaster struck again when on January 10th 1954 G-ALYP – commanded by Capt. Alan Gibson with Capt. Livingstone (BEA) and Capt. V. Wolfson (BOAC General Manager subsidiary companies) and 29 passengers – including the famous War Correspondent Chester Wilmot, crashed into the sea south of Elba shortly after taking off from Rome. It was during the climbing phase and this factor led to much speculation that the accidents could be linked in some way.
20 minutes after takeoff, Capt. Alan Gibson had a conversation with a colleague in an Argonaut departing Ciampino Airport, Rome – the Comet had cleared 26,000ft, no problems were reported. There was no record of severe turbulence. Although there was no clue as to the possible cause, as a precaution, BOAC suspended all Comet services.
Sabotage was considered – witnesses reported hearing explosions. Others concentrated on more practical possibilities such as an explosion of a kerosene-air mixture – possibly somewhere in the wing structure – or an explosion of vaporized hydraulic fluid. Both kerosene/air mixtures and hydraulic fluid vapour concentrations were known to be explosive in certain circumstances. But this was pure speculation.
With the Corporation’s entire Comet fleet grounded, a though examination of the remaining Comets began. The feeling began to grow that the problem could be more fundamental – something not previously considered. But by routine examination it was unlikely that incipient structural failure, if the was the cause, would be detected.
All this attracted wide publicity. It reassured the public that the authorities were determined to discover the cause. Air France and UAT gave their Comets similar examinations. The Canadian Air Force, in contrast – not being subject to civilian constraints – did not follow the lead.
The Comet was not after all the first new civil aircraft to suffer mysterious failures early on in its service life. The DC6 suffered two accidents (one with fatalities) after mysterious fires broke out. These were eventually traced to a fuel immersion pump which, when accidentally left on, caused excessive pressure and fuel to vent into the airflow and onto a combustion heater. For a long time the actual cause evaded the investigators.
The withdrawal of the Comet from service resulted in great difficulties for BOAC. If suspension was temporary, other aircraft could be drafted in to cover the vacant routes. If, however, matters were to drag on indefinitely the Corporation would have a major problem on its hands.
With no indications as to the cause extensive testing of the remaining Comets was stepped up, undertaken by de Havilland, BOAC and the RAE.
The Engineering Divisions inspected all aircraft for structural problems – particular attention being paid to those aircraft having flown the greatest number of hours. The Royal Aircraft Establishment were conducting specific tests on a prototype airframe. In addition BOAC were studying, for example, component failure in one aeroplane and electrical, hydraulic or control systems failure on another example.
Everybody was under pressure to do something. The Company to restore confidence in the Comet. The airline to get the Comet back into service. Politicians and the public for an explanation for the losses.
In the meantime a number of precautionary modifications were adopted for the Mk.1.
Armour plated shields were placed between the fuel tanks and the turbine blades – the fear was that a hot turbine blade could shear and rupture the fuel tank. Modifications were made to allow the battery area to be vented so as to prevent the accumulation of hydrogen gas. Additional fire warning detectors were to be installed in the engine cells. Special metal-braided fuel hoses were used with more frequent routine inspection of them.
Other changes included better engine breathers, another temperature gauge was provided in the equipment bay, additional ducting of cool air to electric motors to prevent overheating, there was the removal of hydraulic drain points, a new relay was provided for the battery change-over.
There were improvements to the ventilation to the rear fuselage under-floor area, and extra smoke detectors were installed at the rear, there was also a reduction of the fin spark gap (designed to discharge static electricity more readily). Also the removal of the rudder trailing-edge strip, provision of drain holes in the rear-spar centre section inspection door, and the reinforcing of the wiring to the ADF aerial amplifier in the aileron booster bay. Another modification was the fitting of larger clips on the booster pump circuits.
A comprehensive list of modifications. But of course most of them were destined for later models anyway, many having been devised as a direct result of operational experience. Once the decision to ground Comet had been taken the opportunity was taken to ‘update’ the aeroplane as much as possible. Hopes were high – few aircraft had been more thoroughly and realistically tested than Comet.
Meanwhile initial attempts to locate the wreckage had failed. The Navy was asked to assist. The Commander in Chief, Mediterranean – Admiral the Earl of Mountbatten of Burma – coordinated a Royal Navy task force. HMS. ‘Wrangler’, and ‘Sursay’ were joined by the salvage vessel ‘Sea Salvor’ and the boom defense ship ‘Barhill’ . Now specialist equipment was available in the form of under-water television, special diving suits, a diving chamber and heavy lifting gear.
By early April an amazing 65% of the Comet had been recovered.
G-ALYW made a test flights with ARB and MCA engineers prior to returning to service
Back into service
On March 12th – after extensive modifications – BOAC G-ALYW made a test flight with ARB and MCA engineers. This was necessary prior to the re-introduction of scheduled services.
Why did Comet services resume? No-one could think of anything else that could be usefully done to increase the safety of the aeroplane and while out of service the aircraft was not producing any return for the heavy financial investment committed to it.
So on 23rd March 1954 the decision was taken by the Corporation, with the approval of the Minister, the ARB and the Air Safety Board, to resume jet services.
Seventeen days later, 8th April 1954, press reports thus –
Early this morning BOAC announced that it had ordered the grounding of all Comet airliners throughout the world.
This decision followed the news that Yoke Yoke, carrying fourteen passengers and a South African crew of seven, was missing. After leaving Rome the crew had signaled that they were “over Naples and still climbing”. Nothing more was heard.
The crash site: the Tyrrhenian Sea, north of Stromboli (NNW of Messina, Sicily). Five bodies and some wreckage was recovered early on. The depth of water in this region was estimated to be around 500 fathoms – far deeper than the Elba crash site.
Certificate of Airworthiness withdrawn
The C.of A. was withdrawn and all Comet operations were suspended.
The only exception was the development Comet 3 which continued its test flying programme. The Royal Aircraft Establishment began a major investigation under the direction of Sir Arnold Hall. Sir Miles Thomas summed up feelings when he said,
“we have got to do some fundamental thinking about the Comet altogether.”
In April 1954 John Profumo, Joint Parliamentary Secretary to the Ministry, made a statement to Parliament announced that a public inquiry would be held into both Comet accidents. The Ministry of Supply would arrange and co-ordinate an exhaustive investigation into, and tests upon, the aircraft. The full resources of his department would be put at the disposal of the investigators. The RAE, Boscombe Down and de Havilland would be heavily involved. Lord Selkirk said,
“It should be recognized that this is not a time for despair, but rather a challenge to the whole engineering and scientific ability of this country.”
All Comets were recalled for investigation and tests
At BOAC maintenance work was concentrated on two aircraft – one with 3510hrs and the other 3471hrs which made them of particular interest since Yoke Peter had 3605 hrs. at the time of the crash.
All stripped out parts were set aside, labeled, and meticulously examined. The engines were removed for separate examination. The frame of the Comet was further stripped of sound proofing and trim to expose the structure both inside and out. Another aircraft had its electrical systems examined in detail.
de Havilland were hard at work too. G-ALYU returned to Hatfield in April 1954 for detailed examination. It too had flown over 3,500 hours so it was of particular interest. At Hatfield the fuselage was modified for under-water pressure testing. However the actual testing was performed at Farnborough where, after the wings had been refitted, the structure was subjected to pressure testing at the same time as wing loading cycles were being applied.
So to sum up the Comets were distributed thus:
G-ALVG was at the R.A.E. undergoing structural fatigue and proof loading tests
G-ALYS – was undergoing a more general detailed examination at Farnborough.
G-ALYX was flown back from Cairo to Hatfield for fuel seepage testing using specially dyed fuel.
G-ALZK which was the second prototype aircraft (G-5-2)
G-ALYT had been fitted with Avon 501 engines for Mk.2 development.
G-ALYW was flown from Colombo to London Airport in April before transfer to Hatfield.
G-ALYP (lost at Elba),
G-ALYV (lost at Calcutta)
G-ALYY (lost at Stromboli)
G-ALYZ (lost at Rome)
G-ALYR was severely damaged when it ran off a perimeter track while taxiing for take-off. Repaired and ferried back to London Airport but was unlikely to go back into service.
G-ALYR after structural testing – © Gordon Riley
G-ALYU in water tank at RAE
Joint test flights
BOAC had bought the second Canadian Pacific Comet (CF-CUM) and re-registered it G-ANAV. This aircraft had strain gauges installed at London Airport and was then transferred to Farnborough for flight-testing.
de Havilland and RAF crews undertook these flights. Leading the flight test team was Sqd. Leader Roger Topp (later famed for his ‘Arrow Nine’ formation when he commanded the RAF aerobatics team).
de Havilland Test pilots accompanied these flights as advisers, for example, as to whether a particular manoeuvre had been performed by the Company or whether they were operating in ‘unknown territory’!
Peter Bois accumulated some eighty hours with this team and described them as skilful and highly professional, “a pleasure to fly with.”
The flights could be extremely hazardous! One unexpected problem that could be encountered was from the ‘bends’. Test aircraft operated un-pressurized – so for most of the flight oxygen was necessary. Some crew members were more susceptible to the bends than others – it was unpredictable and could be related to a particular flight.
Because of this phenomena all the crew members were tested in a decompression chamber – the sort of chamber used by divers. When suffering from the ‘bends’ aircrews could experience varying degrees of discomfort, ranging from mild to excruciating pain.
On one occasion a replacement de Havilland Flight Engineer, who had successfully passed the decompression tests, joined the test flight. Take-off was at 4:20 am to allow four hours of flight before the sun could affect fuel cooling – the object of the test. They climbed to 34,000ft and then a low speed cruise was started. After 20 minutes the F/E was taken ill and clearly was in great pain. He gallantly elected to continue with the flight but he was over-ruled and the flight abandoned. This engineer was to fly at even greater altitudes without mishap.
G-ANAV had been fitted with strain gauges at various critical points on the aircraft. Loads were measured during turns, pull-ups and pushovers at various speeds and at various altitudes. de Havilland had already performed many of these tests themselves.
Peter describes a typical test flight with RAE personnel,
“A number of flights were made through cloud, to check the stresses during turbulence. At the several flight observer stations positioned in the cabin section, instrumentation allowed the stress figures to be observed in real time as well as recorded. A Dr. Burns directed this programme from one of these stations. Brilliantly clever, this lady was equally decorative, but she possessed far more courage than was comfortable!”
After several passes through turbulent cloud, increasing speed each time, Dr. Burns had registered what she regarded as some interesting readings but they were not ‘very high’. She requested that the manoeuvre be performed at some 30 knots faster! The crew dissented having registered some very high readings on the cockpit accelerometer during that previous run! Dr. Burns, scathingly, “Well, if you don’t like it, make it twenty knots, but thirty would give a worth while figure!” Peter, “Much as I admired her, she scared the hell out of me!”
The RAE tests also required evaluation of the accelerated stall. In these the Comet was exceptionally well behaved. When stalled in a 45° turn the Comet’s nose would drop and the aircraft gently roll to some 30-35° in the other direction, before the ailerons became effective. Successive flights were undertaken at increasing bank angles. The RAE wanted to determine whether the Comet had a tendency, as was the case with some other aircraft, to roll to an inverted position in stall recovery. Even with 80° of turn at the stall the opposite roll in recovery, could be stopped at about 65°. Some aeroplane!
Speculation – the cause?
Why had the Comets crashed? There was much speculation as to the cause, for example, it was suggested that some form of catastrophic engine failure may have been to blame for the loss – it was argued that in each case the Comets were climbing on full power. Yet this had not been borne out in tests … the Ghost had been spun to some 17,000 rpm. without failure…whereas in the take-off phase only 10,250 rpm would normally be required – and of course the engines would be throttled back once airborne.
In any case there was no evidence of engine failure in wreckage from the Calcutta or Naples sites. It will be recalled that one of the modifications made before Comet services resumed in March 1954 had been the fitting of amour plated shields between the engines and the fuel tanks on all Comets. It was extremely unlikely that turbine failure could have been the cause.
Another possibility was an explosion – perhaps in a nearly empty belly tank, which could, under certain conditions of temperature and pressure, explode? Was there a possibility of a spark igniting the vapour given off from the hydraulic fluid – which is not flammable under normal conditions – but may be so under exceptional conditions? Both phenomena were known. The ability of hydraulic fluid to ignite was demonstrated by an incident that occurred during the testing programme.
On this occasion Peter Bois had been involved in a series of acceleration/stop tests. The disc (the Comet was the first aircraft to have disc brakes) became so hot that when leaking hydraulic fluid was accidentally sprayed on to it caught fire.
“In the cockpit, we noticed nothing, braking was completely effective and it was only the arrival of a rapidly dispatched fire wagon and a call from the Tower that stopped us from continuing the tests.”
The Comet was also the first civil aircraft to be pressure re-fuelled. Could this have been the problem – at a rate of 300 gallons a minute could the aircraft be accidentally overloaded? It was believed that this was exactly what happened to an RCAF 1A on a training flight.
Some pilots speculated that with fully powered flight control systems a lack of feel could result in the pilot not being able to sense the excessive loading on the airframe (from excessive stress for example) and, by over correction exacerbate the problem?
In the early part of the investigations failure of the pressure cabin was not considered a strong possibility. It was recognized that if there had been a sudden decompression – and the passengers had blacked out – the pilots were provided with there own oxygen masks which were triggered automatically on decompression. They at least should have been OK. In fact there had been many reported examples of sudden decompression at 20,000ft – particularly after windows or astrodomes had failed in flight – and usually a safe landing had been made. However none of the Comets had been at cruising altitude when disaster struck.
BOAC Fleet Problem
In early May production of the Mk.2 Comet was suspended – as was work on the prototype Mk.3.
All the modifications appeared not to be been sufficient to prevent further losses. So the Company had no choice but to await the findings of the official investigations and the conclusion of the Inquiry.
Meanwhile BOAC found themselves acutely short of aeroplanes. They were forced to suffer the embarrassment of having to negotiate with rival carriers in order to offer their passengers seats.
Piston aircraft were urgently needed to provide carrying capacity pending the arrival of the Britannia. Discussions took place with South African Airways and with Qantas Empire Airways, with whom the Corporation was negotiating, to loan from them a number of aircraft. Qantas were already operating Lockheed Constellations. BOAC had ordered eight Super Constellations and fortunately deliveries were just beginning. It would make sense for BOAC to lease aircraft of a type compatible with those they were purchasing and thus economize on crew training.
To purchases American produced aircraft required Governmental approval because of Dollar exchange considerations. The Government indicated that it would, in the circumstances, be sympathetic to further purchases of Constellations, DC6s and DC-M4s.
The results of all the investigations – both by the RAE and de Havilland – were presented to the Court of Inquiry which met in the Autumn of 1954.
Cost of the losses
So effectively the accidents killed off the Comet 1 and 1A and the Comet 2 as commercially. But what was to be the fate the existing aircraft? … and on what basis would compensation be paid? This was answered in 1956, when a statement was made to Parliament, by the Minister for Aviation.
He explained on what basis prices were assessed when the Comets were withdrawn from service. He said that the purchases were based on ‘an assessment of future requirements’, the statement said, “it was decided that the prices to be paid could be settled by normal negotiations without taking advantage of the Company’s vulnerable position to drive the hardest possible bargain”.
The Ministry thought that, “to do otherwise would have been inconsistent with the purpose of the arrangements, namely, to safeguard the public interest in the Comet and other projects which the de Havilland Company had in hand, and to enable BOAC to acquire Comet Mk.4s in place of the Comet Mk.2s which were on order at the time of the disaster.”
What was interesting was that the figure quoted for value of orders for Comet Mk.1s and Mk.2s at the time of the grounding was given as £10,000,000. Whereas a price of £260,000 was paid by the Ministry for the five stripped airframes used for routine tank tests. The total cost of salvage in the Elba and Naples accidents was put at £432,301.
In addition details were given of arrangements for completing, or otherwise, of the Comet Mk.2s originally destined for BOAC.
Eighteen Mk.2 airframes were involved thus-
10 modified Mk.2s were destined for RAF Transport Command. These were required to obtain an ARB certificate confirming a minimum flying life of 5000 hours per aircraft operating at full cabin pressurization. These Comets were to have full Certificates of Airworthiness by the ARB and so could carry passengers if required.
3 unmodified aircraft were allocated for other duties in the RAF which were required to obtain ARB certification confirming a minimum flying life of 2000 albeit at a limited pressurization.
3 unmodified, incomplete aircraft for fleet reserve (spares).
The 16 aeroplanes listed above cost the Government a total of £4,068,000 which included £1,188,000 for sixty-six Avon 200 series engines for use in the Mk.2s.
In addition two more examples were required, at a cost of £555,000: one a modified test specimen for testing in pressure tanks and one a modified aircraft for routine tank testing.