RETURN TO SERVICE - FULL STORY
CHAPTER 5
 Testing Begins

Fox-Bravo and the Engineering team pose for a photograph before the aircraft heads to Istres
With British Airways beginning to fit the Kevlar liners to Concorde Alpha Foxtrot, Air France rolled out Concorde F-BVFB from the hanger at Paris CDG airport. Over the past weeks engineers had been working to bring her back up to flight status. This would allow her to fly sub-sonically to start the tests that would hopefully return Concorde to passenger service.

Fox-Bravo would take part in the first set of tests that had been designed by the manufactures to prove that any incidents that were to occur in the future after the proposed modifications had been put in place would be of no danger to the aircraft. Fox Bravo would fly to the French Military test base at Istres, where high-speed taxi tests could be conducted on the long runway there.

During these high-speed runs coloured water would be pumped out from locations under the wing to simulate fuel leaks. This would in theory allow the engineers to understand where leaking fuel would flow to at high speed and what effect, if any, it would have on the aircraft if any proportion of this were to be ingested into the engines.

Enthusiasts gathered to help send the aircraft off from the VIP stand at CDG, Paris.
On the 18th January 2001 Concorde F-BVFB was ready to depart from a very misty Charles De Gaulle airport to head for the South of France. The aircraft had been towed, in the early morning, to Stand Sierra 11 - the VIP stand at Charlese De Gaulle - to prepare for departure

Scores of Concorde enthusiasts and media representatives lined a fence near the runway. While readying the aircraft for takeoff, the pilot, Edgard Chillaud (AF Concorde division chief), leaned from the cockpit window holding a video camera and waved to the onlookers, who held up a banner reading "Concorde, We Love You," against the airport security fence.

This was to be the first departure of Concorde (CDG) from CDG since the accident 6 months previously. Many people were anxious, including the mayor of Gonesse who had said the aircraft should not be flying. Precuations would be taken to ensure that the flight path of the aircraft would not overfly the suburb town and she was given runway 09 right to use.

Fox Bravo departs into the mist from Paris CDG airport (Reuters)
Half a dozen fire tenders were stood by the runway, along with a large contingient from the media who were there to see the flight depart. The same members from the world's media had been at an Air France press conference the previous day, in the Airport's Hilton Hotel, where the airline had laid out their plans for Concorde, including the test that F-BVFB was about to take part in.

Concorde Fox-Bravo, with callsign AF370V, taxied along the MIKE taxiway on the way to the runway holding point for the departure. Christine Lacroix, the Airport tower controller on duty, spoke to Captain Edgard Chillaud" I love my old Concorde" Chillaud, hearing her voice answered "Merci ! Christine !!!".

After the unofficial formalities had been completed, Christine in the tower give the aircraft take off clearence. At 11am Concorde was back in the sky above the French Captital, for the 50 minute subsonic transit flight to Istres.

The specially installed pipe work to let the test fluid escpae from locations under the aircraft's wing
On arrival at Istres the aircraft was fitted with a take that could contain up to 50 litres of coloured water. A system of pipes were fitted to the underside of the wing so that this water could be pumped out under a pressure of 15Bar to three places around the engine 1 and 2 intakes. One of these places was as close as investigators could get to where they felt that the main fuel leak during the accident had happened.

Although the new wing tank liners are designed to stop or significantly reduce fuel leaks in the event of a puncture, minor fuel leaks nevertheless must be scrutinized to confirm that they would be thin enough to suppress risks of inflammation.

The release control and observation / recording position in the rear cabin of Fox-Bravo
Under command from a specially mounted test position in the aircraft's rear passenger cabin, various exit points could be opened at different speeds up to a maximum of 205 kts, well above Flight 4590's 150kt, V1 rotation speed, according to Air France's Concorde chief pilot Captain Edgard Chillaud.

The aircraft's weight, during the test, would be no more than 110 metric tons (242,000 lb.), which is significantly below Concorde's maximum takeoff weight of 408,000 lb. To provide a thorough understanding and allow post-test analysis of what was occurred, 3 small video cameras were mounted on the left undercarriage leg to monitor the tests. The cameras could also be viewed at the monitoring station in the rear of the aircraft.

The test at Istres took place under the supervision of Didier Ronceray who was Airbus' Flight test engineer. Pierre Grange was in charge of the aircraft as Test pilot, along with him was Air France's Edgard Chillaud as co-pilot and Roger Beral as Flight engineer.

A test run at Istres with the fluid flowing from under the wing
The first set of tests were scheduled to begin on the afternoon of 23rd January with the aircraft at the set weight of 110Tonnes. To ensure that test runs at the critical speeds of between 130 and 180knts could last as long as possible, reheat was not selected so as to slow down the rate of the aircrafts normal runway acceleration, thus giving maximum time in the required speed range.

Through work in the Concorde simulator at Charles De Gaulle, the flights crews had come up with a procedure that would allow then to rotate the aircraft to an angle of 6 degrees for a few seconds between speed of 120 and 160kns. Tests under these conditions were of interest to the manufacturers, as F-BTSC had rotated during the accident when the fuel was leaking. This would provide further data on what was occurring with the leaking fuel.

A close up shows that the fuel leaking from certain locations could enter the engine air intakes
The tests were concluded on the 1st February with a test at 205kts, with the engineers happy that they had all the results they needed to back up their current computational data, that would allow them to understand what had happened with leaking fuel. It would be up to the team at BAE Warton, in the UK, to try to understand how this leak had been ignited.

F-BVFB flew back to Paris on Saturday 3rd Feb 2001 arriving back at 15:00GMT. To allow air France to keep their crews familiar with the aircraft, a new crew were allowed to fly the aircraft home. They were:

The first results of the test seemed to confirm that leaking fluid from an area similar to where the tank failed at Gonesse would indeed flow close by or into the engine's via the air intakes.

Two major tests were scheduled to take place in the UK during the early part of 2001: Rolls-Royce needed to prove to the aviation authorities that the Olympus engine could cope with a small amount of leaking fuel or hot gasses being ingested into it.

A Concorde Olympus engine on the test stand beofre the fuel ingestion tests got underway

To most people's surprise the original engine test stand at Shoreburnesss in Essex, east of London, was still in existence from the engine commissioning and development in the sixties, seventies and eighties. This would be used to understand how the engine behaved in surge conditions.

British Airways "donated" one of their 38 Concorde Olympus engines, that had been put together with time expired or high life parts, to be tested to near enough destruction. The engine was mounted onto the freshly painted test stand that had been unused for nearly 25 years, and had a simple air intake fitted onto it.

After an initial hiccup the test proved very successful and proved the analysis of the accident in Gonesse to be true, where many engine surges were seen to have occurred.

Tests with a 0.5lt "slug" of fuel introduced at high velocity produced a surge, which resulted in some engine damage. Further tests showed that with fuel in the form of a spray, which is more representative of a leak, the engine would accept flow rates as high as 1.6lt/s without surging, a very good performance for an engine whose design had started in 1949.

With higher flow rate with those approaching the levels that were seen during the accident in Paris, the engine initially surged but then recovered proving that it could easily cope with any fuel leaks that could occur after the fitment of the internal tank liners.

The fuel ignition test rig at BAE system's Warton facility (BAE Systems)
The second major test was carried out at BAE Systems' Warton factory where they recreated part of the left wing, undercarriage bay, landing gear legs and air intakes to try to understand where and how the fuel was ignited and how the fire burned. To simulate the airflow during an average take off the rig was built close to the exhaust from Warton's high-speed wind tunnel. A hole, known to many as the "Gonesse hole" was placed in the rig, that would allow fuel to be released into to the landing gear area at around the same flow rate that occurred during the accident.

The engineers believed that there were three likely sources of ignition: an engine surge, electric spark or from the engine's afterburner or reheat. From the measurement of the un-burnt fuel stains on the runway and the soot deposits from the burnt fuel it was determined that the time between the leak occurring and the ignition was around only one second.

Each of the three ignition theories were individually tested :

Engine surge

To simulate a surge, a 70 millisecond explosion was detonated and projected out and forward from the intake. A surge from the auxiliary intake of the no2 engine did cause the fuel flowing in the landing gear area to ignite, but this was ruled out of the Gonesse accident as the surge was recorded to happen after the fire had started from data analysed from the flight data recorder. This and other subsequent surges were probably cause by hot gasses being ingested by the engine.

Spark from landing gear power cables

 
Fuel leaking from the Gonesse hole is ignited by a spark in the gear bay (BAE Systems)
In the gear bays there is a 115 volt supply that runs to power the cooling fans for the carbon brakes. It was suggested that the debris from the tyre bust could have damaged this cabling causing it to arc.

In the areas of the suspected damage a spark was created by a set of 4 igniters that each produced a 3 Joule spark randomly. During this test as soon as the igniters were activated a fire broke out within less than a second. The fire became established in the wake of the undercarriage leg and side stay. The patterns of the burning fuel closely resembled that in the pictures of the stricken aircraft taken on July 25th 2000.

Reheat

The fuel was released and allowed to propagate the length of the engine bay to the area where the reheats would be situated. Gas burners were put in place to simulate the reheats. The fuel was ignited on this occasion, but was unable to move forward in the airstream. The fuel was flowing at around 106m/sec but the fire could only advance at 10m/sec therefore completely ruling out the reheats as the primary ignition source.

Although the engineers at Warton could not be 100% certain, they felt that their best educated estimate was that the ignition was caused by a spark from arcing in the landing gear brake cabling. It was proposed that this would be armoured in the remaining aircraft.

Towards the end of February the Anglo-French working group met for the fifth time and were for the first time "optimistic" that the work being done would allow Concorde to return to passenger service. "The Work achieved up to now makes it possible to be optimistic on the re-establishment of the flight certification of Concorde", was the view the general Management of the French Civil aviation regulator (DGAC).

The working group however noted "The need to have the time to go over the results and analysis of the tests used to validate the modifications that are being considered, and after this to allow installation of the modifications on each Concorde individually", according to the official statement, which drew up a list of tests that still had to be accomplished.

Tests on the ground and in flight with a Concorde from British Airways equipped with tanks reinforced by a flexible internal protection in Kevlar-rubber will take place " in the next weeks ". The main function of these tests will be checking the correct operation of the system of calibration in the tanks that have been modified.

work underway to Complete F-BTSD's D check with parts from F-BVFF, sitting behind, being used. (Max Kingsley-Jones)
At the start of February, with the confidence that Concorde could be re-certified, Air France moved two of their aircraft from storage outside on the airport tarmac into the technical hanger at CDG. Work also recommenced to complete work on the D check on F-BTSD, that was nearing completion when the aircraft were grounded, with parts being removed from Concorde F-BVFF to enable Sierra Delta to be delared servicable.

These tasks would allow engineers to shortly begin the preliminary work for the modification programme. This programme of preliminary work included the strengthening of the No.72 rear wing spar. This was remedial work that was required before the crash had occured. Small stress related cracks were discovered in the rear spars of both airline fleets after a routine inspection on a British Airways aircraft, when the secondary nozzle assembly had been removed during on going maintenance.

Air France's planning was to have 4 aircraft in commercial service with the 5th cycling through routine engineering maintenance. British Airways planned to modify all 7 aircraft, and do them 2 at a time after the initial aircraft had been successfully tested.