Registration
Peter Bois [quotes in red] recalled that in the course of development very few faults were discovered.
The prototype satisfied the requirements of the Air Registration Board – who, it must be added, were also attempting to write their own manuals on ‘pure-jets’, literally, as they went along.
On one occasion though there was a minor difference of opinion between the two parties. Dave Davies (ARB Chief Pilot) was extensively involved with the Comet certification program. Peter Bois described him as, “an exceptional pilot.”
But on this occasion he requested that the Company modify the fire warning bell as he considered that it was not loud enough! The DH pilots thought otherwise.
At the next ARB flight the flight engineer was primed to set off the fire warning bell during take-off! The ARB Chief Pilot was comfortably settled at the controls. Peter continues, “everything proceeded normally, until I called “V1” (decision speed) when, as arranged, the fire bell was set off. Amid loud ringing, To his credit Dave took it pretty calmly and called ‘Engine Fire – identify engine – fire warning checklist’. He did however look rather relieved when I answered ‘Disregard – just checking your hearing aid!’ ‘You bastard’ he replied.”
The request to make the fire bell louder was dropped!
Flutter
One incident that could have been more serious occurred after the Comet had long since passed all the flight tests designed to produce flutter. Peter comments, “Without warning, we got it in a big way! The Comet’s Vne was 300 Knots IAS, but the ARB required a demonstration to 367 Knots IAS or thereabouts for various tests.”
On one of these test flights the aircraft was a few knots below its maximum, “when flutter started, the instruments became a blur and break-up seemed imminent. John Cunningham, in the left seat, immediately closed the throttles, while I extended the speed brakes. The aircraft seemed to take an age to decelerate and only stopped shaking at 270 Knots.” All were shaken.
It was not possible though to tell whether it had been rudder or elevator flutter that had been induced. A series of test flights were undertaken with the aircraft fully instrumented in an attempt to reproduce and identify the source of the problem. Peter explains, “Mach and altitude were explored, but it only occurred at a particular Centre of Gravity. It was then realized that this C. of G. resulted in there being zero elevator hinge moment, i.e. under this exact condition you could, theoretically, remove the elevator altogether and still be in trim. As a result there was no aerodynamic damping to prevent flutter starting. This led to the conclusion that the existing mass balance, which was not directly connected to the elevator but to the servodyne linkage, was in no way doing the Job! How fortunate that the Comet had power controls, which had the effect of limiting the amplitude of flutter. A manual controlled elevator would certainly have broken away and the aircraft lost.”
The problem was solved temporarily by the attachment of an external balance weight. The final solution on production aircraft was an arm and weight fitted directly to the elevator torque tube within the tail-cone fairing.
Comet Unit
In July 1950 the second prototype G-5-2 with John Cunningham and Peter Buggé made its maiden flight and was delivered – as G-ALZK – on 2nd April 1951 to the BOAC Comet Unit at Hurn Airport for a ‘500 hour crew training and route proving program’. During this time, in addition to fuel consumption tests, techniques of holding and descents were extensively evaluated.
Twelve long distance proving flights were made to destinations such as Johannesburg, Beirut, Delhi, Djakata and to Singapore. These flights were not only intended to evaluate the Comet on the longer BOAC routes but were also used to ascertain the need for, and facilities available at, a number of re-fuelling points en-route. The Comet required BOAC to re-think its established handling procedures and re-assess the turn-around times before they could attempt to schedule services
Faults
The Comet had little difficulty in satisfying the ARB and being granted it’s Certificate of Airworthiness.
Often, in the early days, it was a scramble to get the aircraft into the air for a series of test flights because some component or other was not yet available – the majority of Comet components were manufactured ‘in-house’ – although some specialist items would be supplied from an external sources.
On one occasion during a flight the crew detected the unmistakable smell of hot electric’s. A frantic search ensued. After a while smoke began to curl out from below the windscreen pillar – the problem was eventually traced to a faulty wiper motor. It turned out that this particular motor was a ‘one-off’ having been disabled with a locking pin, because it did not work too well! Someone had accidentally knocked ‘on’ the switch so the wiper was trying to operate while being prevented from doing so by the pin! The motor had over-loaded!
On another occasion the DH team were performing in-flight re-light tests on the Ghost engines. On one such test some insulation material located between the engine jet-pipes filled with fuel and ignited. Fortunately one of the flight observers saw the flames and alerted the crew. In this case the fire burned itself out – with only minor damage to the surrounding structure. Following this incident there was a rapid redesign of the fuel drainage system so preventing fuel leaking into this critical area.
Standard practice is that when a problem of this nature occurs, the cause is properly identified. Where there are possible safety implications – the Company would routinely notify the Ministry of Supply of the facts so that they could pass on details to other manufacturers. Sadly, not long later, a similar fire broke out on a prototype Valiant bomber which resulted in its total loss. In the case of the Valiant the fire could not be seen from anywhere onboard and continued to burn unchecked until it caused a major structural failure and a subsequent loss of control.
On one occasion G-5-2, the second prototype, was being tested in a high-speed dive. Suddenly there was a loud bang and then nothing else to indicate a problem – no unusual noises or buffeting. Later it was discovered that a panel measuring some 6ft by 2ft from the top of one of the engines had broken away! The panel was subsequently returned to de Havilland by a farmer into whose field it had fallen! The cause of this failure was self evident – a complete line of rivets was missing! Amazingly the aircraft had already flown several hundred hours and at higher speeds than on the day of the panel loss.
Incidents
Flight-test crews generally accumulated far less hours than their airline colleagues but were on duty whenever the development program demanded – so many flights at the weekend? As compensation of course test flying was far more varied, much more exhilarating and often, much more hazardous.
Peter gives us an indication of what test flying was all about, while telling us something about Comet development, with the following anecdotes:
“When the Comet was designed hot air ducts were provided in the leading edges of the wings to give some protection against ice build-up. The second prototype had its temperature distribution mapped and then sensors were installed to measure the rate of ice accretion. The crews task was to find suitable ‘icing’ conditions so that measurements could be made.
“This was easier said than done. Even with the help of distant Air Traffic Controls – who would relay reports of icing conditions coming in from other aircraft – it was often difficult to find the right conditions. They would literally scramble the aircraft when a positive report came in and tear off to the reported location – they would find themselves practically anywhere in Europe seeking suitable atmospheric conditions. On one occasion they were as far south as Marseilles but they were then diverted out over the Atlantic and then up to the Arctic circle. Amazingly nothing more than light icing could be found.
Peter Bois, “We even resorted to doing steep turns inside building cumulous clouds! During the first turn we sometimes got quite a respectable rate of accretion, which would cease just as we completed the 360 degrees.”
It was concluded that the icing condition was of very short duration.
After some eighty five hours of fruitless search, the team finally got their only interesting results! They’d concluded, by this time, that swept wings were not inclined to pick up ice in the way that straight wings do and that the existing de-icing system was effective.
“On this last flight we simulated a holding pattern at about 16,000ft in strato-cumulus cloud. Power was maintained with the outboard engines, the inboards idling for maximum economy. When power was increased on the inboards, after about 1½ hours, heavy vibration started. It was evident that ice had accumulated on the compressor blades and some of it had broken off, unbalancing the rotating assembly. Descent to an altitude below freezing level allowed the ice to melt and run back into the engines, which could then be brought up to climb power. Clearly some further research was needed into this phenomenon.”
“A structure was designed with a number of nozzles, so that a fine spray of water could be directed over the whole area of number three engine air intake. The accretion rate could be varied by adjusting the water pressure and the temperature varied with altitude. A periscope from a midget submarine was modified so that it could be extended from inside the cabin, thus allowing the inspection of the engine intake after ice had been allowed to build up. This set up was ideal because it allowed tests to be done in summer as well as winter and for as long a time as required”
The results of these tests was that it enabled a minimum recommended rpm be set for the Ghost engine when idling in icing conditions. Experience gained with the Ghost was applied later in the development of the Comet Mk.2 with Avon engines when a similar test rig was used.
Peter, “The tests also showed that it was essential that no ice should be allowed to form on the engine intake. During test-bed engine runs, Company engineers had satisfied themselves that the Ghost could ingest large amounts of ice, without damage, by throwing buckets of ice cubes into the intake. They were proved wrong when, towards the end of the spray-rig tests on the Ghost, a fairly large piece of ice, which had been allowed to form on the intake, slipped through the compressor and completely stripped the guide vanes between it and the combustion chambers. The exhaust gas temperature shot up over the limit within seconds and the engine was shut down, as quickly as possible”.
Following this, minor modifications were made to provide better de-icing to the air-intakes to prevent significant ice build-up.
Peter Bois,
“We even resorted to doing steep turns inside building cumulous clouds! During the first turn we sometimes got quite a respectable rate of accretion, which would cease just as we completed the 360 degrees.”
It was concluded that the icing condition was of very short duration.
After some eighty five hours of fruitless search, the team finally got their only interesting results! They’d concluded, by this time, that swept wings were not inclined to pick up ice in the way that straight wings do and that the existing de-icing system was effective.
“On this last flight we simulated a holding pattern at about 16,000ft in strato-cumulus cloud. Power was maintained with the outboard engines, the inboards idling for maximum economy. When power was increased on the inboards, after about 1½ hours, heavy vibration started. It was evident that ice had accumulated on the compressor blades and some of it had broken off, unbalancing the rotating assembly. Descent to an altitude below freezing level allowed the ice to melt and run back into the engines, which could then be brought up to climb power. Clearly some further research was needed into this phenomenon.”
“A structure was designed with a number of nozzles, so that a fine spray of water could be directed over the whole area of number three engine air intake. The accretion rate could be varied by adjusting the water pressure and the temperature varied with altitude. A periscope from a midget submarine was modified so that it could be extended from inside the cabin, thus allowing the inspection of the engine intake after ice had been allowed to build up. This set up was ideal because it allowed tests to be done in summer as well as winter and for as long a time as required”
The results of these tests was that it enabled a minimum recommended rpm be set for the Ghost engine when idling in icing conditions. Experience gained with the Ghost was applied later in the development of the Comet Mk.2 with Avon engines when a similar test rig was used.
Peter, “The tests also showed that it was essential that no ice should be allowed to form on the engine intake. During test-bed engine runs, Company engineers had satisfied themselves that the Ghost could ingest large amounts of ice, without damage, by throwing buckets of ice cubes into the intake. They were proved wrong when, towards the end of the spray-rig tests on the Ghost, a fairly large piece of ice, which had been allowed to form on the intake, slipped through the compressor and completely stripped the guide vanes between it and the combustion chambers. The exhaust gas temperature shot up over the limit within seconds and the engine was shut down, as quickly as possible”.
Following this, minor modifications were made to provide better de-icing to the air-intakes to prevent significant ice build-up.
Flexible Loading?
If more flexible loading could be achieved it would benefit the operators economically. A series of test flights was requested by de Havilland to evaluate increasing the forward centre-of-gravity (CG) by a small amount. These tests were hazardous and commenced by determining existing limits by checking the elevators effectiveness on landing. The objective was to provide guidelines for BOAC and to simulate conditions they might encounter on some routes where approach lights were often inadequate or where, in poor visibility conditions, the aircraft may be above the ideal glide path when visual contact with the runway was achieved – necessitating a rapid final descent.
Peter Bois, “One should be able to close the throttles at 300ft above the runway, steepen the approach path to maintain the correct approach speed for the weight and still have enough elevator control to flare for a normal landing. Tony Fairbrother was the flight observer on this test and I asked him to ballast for the existing forward limit to start with. I hadn’t done the original tests and wanted to do a couple of landings to get the feel of it before trying anything new. After the second landing, I was convinced there was nothing more to come and suggested we abandoned the idea of going further forward with the CG”.
(The CG being tested on this flight was not the operational one, such as the airline would use, but an ‘extended’ limit – ‘limit’ being the operative word because this was the limit quoted in the manual as the maximum figure which provided for a suitable ‘safety margin!).
However Peter was persuaded to try a landing with a compromise revised CG setting – roughly half that requested by the Company.
“As soon as I started the flare I knew we’d gone too far and was prepared for the almighty crash, with which we hit the ground.”
It was agreed that the limit had been determined previously. The aircraft did not appear badly damaged but needed a ‘hard landing’ inspection.
“On leaving the aircraft we saw ominous wrinkles in the fairing between the upper wing surface and the fuselage. This was thin gauge sheet metal and did not contribute to the aircraft’s strength, but it was nevertheless, a sure sign of an overload. A very thorough inspection was initiated. including the removal of the main landing gear. The only thing found was that the main attachment bolt holes had become oval by about 1/100″. The flight recorder’s accelerometer showed that the aircraft had exceeded the maximum ‘g’ by one-two percent and therefore the Stress Department’s calculations had been exactly correct and the damage was simple to repair.”
Stall Characteristics
Stall characteristics are extremely important. As will be described later there was some controversy about the stall characteristics of the Comet on, or near, the ground – the controversy arose after a couple of accidents in service. The point was that the Comet was significantly different operationally from any other aircraft in service. Obviously during the test program stall characteristics were extensively evaluated over a great many test flights.
Peter Bois took part in some of those flights.
“After relatively little development the Comet had excellent stall characteristics. Of course, the ARB required the stall behaviour to be checked over the whole weight range, at all centres of gravity, power on and power off, gear up, gear down, clean (no flaps), take-off flap, landing flap and at low and high altitudes. In addition to stalls in level flight, accelerated stalls i.e. at various bank angles, had to be done”.
The permutations and combinations were endless.
de Havilland test pilots liked nothing more than to impress ‘visiting’ crew who often joined the more ‘routine’ test flights. They were extremely confident too! A pilot from an American airline had been invited to take the right seat and was clearly impressed by how dark the sky became when the aircraft climbed above 40,000ft. On this particular flight the crew had planned to do some stalls at low weight and aft CG on the descent between 20,000 and 10,000ft. Unfortunately the weather deteriorated as the aircraft met a frontal system and they entered cloud at 22,000ft.
“Our visitor was speechless, when I started a series of stalls in different configurations, all on instruments.”
When Concorde was introduced there were very few military aircraft that could catch it and fewer still could maintain Concorde’s speed – but then only for minutes at a time! When Comet was introduced a similar situation arose – a civilian aircraft was far faster than the majority of military aircraft!
On a demonstration flight for Air Chief Marshal, Sir Ralph Cochrane, the Comet demonstrated this. An interception by Gloster Meteors had been laid on. The Comet was cruising at about 35,000ft when a Meteor managed to pull along side.
“John Cunningham increased power to maximum thrust and put the nose down a little. The pilot followed but, very soon our speed exceeded the Meteor’s critical Mach number and it started to get wing drop. He struggled to hold station for a short time, then had to pull away, shaking his fist in fury.” The pilot of the Meteor was unaware of the identity of the VIP passenger who found the incident highly amusing!
Certificate of Airworthiness
The world’s first Certificate of Airworthiness (CofA) for a commercial jet passenger aircraft was granted on the 22nd January 1952.
During January simulated passenger operation flights were undertaken to South Africa so that flight deck crew, cabin crews, and the aircraft handlers could familiarise themselves with the Comet.
G-ALYP (known as Yoke Peter) was delivered to BOAC on 8th April 1952. The last of the line G-ALYZ was delivered on 30th September that year.
Airlines now began to sit up and take notice of the Comet. Overnight, it seemed, BOAC’s rivals had been disadvantaged. They could not compete with slower, less glamorous aeroplanes – the only solution was to offer jet services too.
Carriers were soon converting their options into firm orders – the sales drive had begun. Some airline operators had more foresight than others. In December 1949, following the SBAC show of that year, Canadian Pacific ordered two Comets – in this case opting for an up-rated version – the Comet 1A.
Early on in the Comet program it had been proposed to offer an up-rated version of the Comet. It was to be powered by a development of the Ghost 50 Mk.1 engine. Equipped with four 50.Mk.2s, it had a greater all-up weight and increased full capacity (6,909 Imperial gallons) this version – designated the Series 1A – it was ideal for Canadian Pacific’s Vancouver-Hong Kong route. The 1A also attracted other orders – one each from Air France, U.A.T., and two more for the Royal Canadian Air Force. Panair do Brasil joined the queue placing orders for four Comet 2’s, which too had just been announced, they also negotiated options for two Comet 3s.
Now the problem for de Havilland was one of production capacity. Shorts of Belfast were given work on sub-assemblies and a second assembly line was set up at de Havilland’s Chester factory.
In Service
At 3:10 p.m. on 2nd May 1952 BOAC, with Yoke Peter, inaugurated the worlds first fare-paying jet passenger service – only seven years after the design specification had been finalized. This was a fine achievement.
The service was from London, Heathrow to Johannesburg via Rome, Beirut, Khartoum, Entebbe and Livingstone and was completed in 23 hrs. 37 minutes. After 6,724 miles (10,810 Km) it arrived three minutes ahead of schedule! Crew: Captains A. M. Majendie, J. T. A. Marsden and R. C. Alabaster and First Officer was Ken Emmott.
G-ALYP
By mid-May route proving and crew training had extended to the Far East. In August 1952 services began to Colombo – the journey taking 21½ hours. Singapore services began in October 1952 and, on 3rd April 1953, a service was opened to Tokyo. The 10,200 miles covered in just over 33 hours. Previously it had taken 86 hours!
The Comet 1 operated at an amazing 89 percent of maximum possible load. Generally it cut flying times by fifty percent.
It received Royal patronage in May 1952 when Queen Elizabeth the Queen Mother and Princess Margaret, along with their hosts Sir Geoffrey and Lady de Havilland, enjoyed a four hour ‘leisure’ flight round Europe. The following year they used Comet to attend the Rhodes Centenary celebrations at Salisbury, Southern Rhodesia.
With its superb appearance and performance the Comet became an over-night success with crew and passengers alike. It attracted the sort of public interest that only Concorde would attract today. It is easy to forget how revolutionary Comet was, and how modern and sleek it appeared compared to its contemporaries.