FLUG REVUE December 2002: Boeing 747-400ER nears certifaction

BOEING 747-400ER CLOSE TO CERTIFICATION

By Sebastian Steinke

Edwards Air Force Base in the Californian desert. The test aircraft, registration "N747ER", stands loaded right to the limits of its maximum permitted take-off weight at the start of the runway, engines running. The brakes are released and, with the engines at full thrust, the jumbo sets off down the runway as if for take-off. The giant four-engined jet races down the air strip at up to the maximum taxiing speed of 172kt (318km/h), but the nosewheel does not lift off. For instead of "Rotate", the command is now "Abort", and the dramatic "Maximum brake energy refused take-off test", the test of maximum braking power during an aborted take-off, now commences.

Without reverse thrust, but simply with spoilers and manual brakes, the two Boeing test pilots have to bring their mighty charge, the biggest passenger aircraft in the world, to a halt as rapidly as possible. And this is with brakes that are worn right to the very limits of what is permissible. You can tell that they are working overtime from the white heat they give off before the plane grinds to a halt. Despite all this, in a matter of seconds the heavily loaded jumbo is standing safely on the runway. In fact it has halted 300m sooner than was planned. But its smoking tyres are still heating up, until one after the other their "plugs” (safety valves) blow and release the dangerous excess pressure into the air before it can cause an explosion. Once all the plugs have blown, then the aircraft is deemed to have passed the test and the airport fire fighting service which is on standby is finally allowed to cool down the brakes with water. After a complete change of tyres and brakes, the jumbo is ready to fly again just one day later.

Boeing 747-400ER

Normally manufacturers play their cards close to their chest during testing campaigns. But for readers of FLUG REVUE, 747-400ER Program Manager Kurt Kraft and Flight Test Manager Art Fanning of Boeing were prepared to make an exception. First of all Fanning ran through the flight test programme in outline. Starting with the maiden flight, first of all the basic aerodynamic properties are checked. "The ER is very similar to a normal -400 apart from small structural reinforcements in the wings,” he says. "There are no surprises here. The flight tests simply serve to provide rapid confirmation of data that is already known.”

The programme therefore quickly proceeds to flights in which the secondary control systems are switched off ("limited handling”). These are followed by structural static tests through to tests at two-and-a-half times the gravity of the earth at various altitudes, during which the engineers induce flow separations with special spiral turns.

Finally the flutter characteristics have to be examined. This requires that the aircraft is loaded in an extreme way, for example, so that the stern is extremely heavy, and then flown above what is normally the maximum operating speed ("Vmo”). "We always have a pretty accurate idea in advance of the range in which flutter will occur. We then take the aircraft to that range,” explains Fanning.

The violent oscillations which then set in must not have a destructive effect on the structural strength or intensify. Cameras, especially in the tail area, and a number of special test instruments that are linked up with special, orange-coloured marked cables ("test cables”) in the cabin register the structural damping characteristics of the aircraft. Even though such tests are never totally free of danger, the crew members do not have any parachutes with them. Nor are there any extra emergency exits or ejection hatches. The aim of the test is to demonstrate that the ER has the same flight and handling characteristics as the present 747-400, so that airline pilots currently rated for that model will find the new variant no different to fly.

"Normally the crew consists of the two test pilots, a test director and ground crew. Then on top of them there are the various specialists who have designed particular subassemblies plus additional observers,” explains Program Manager Kraft. Usually there are around 20 people on board, but if the flight is an extended series of tests, there could be as many as 90 passengers.

The majority of all certification flights are being carried out on the two first examples of the passenger version. They are referred to internally as "RF001” (currently in Boeing paintwork) and "RF002” (in Qantas colours). The extra tank in the forward cargo bay of the 747-400ER passenger version gives it 830km more range, although it has had to shed certain installations. For example, the fresh water tank and parts of the air-conditioning system.

The ER freighter version, by contrast, is relatively similar to its -400F cousins and does not have an extra tank. Its strengthened undercarriage can be tested on the passenger version. The first 747-400ERF (in Air France colours) is currently undergoing extended acceptance testing, in the course of which some special freighter options, for example an electronic system for determining the centre of gravity, will be tested.

After testing of its climb performance and flights in which one or more of the engines are shut down, the plane is then subjected to the braking test described above, which Fanning describes with relish as "two minutes of high drama”. The test team approaches the extreme braking test methodically, proceeding one small step at a time. First of all the brakes are precisely calibrated and tested on a series of trials at steadily increasing speeds, with the spoilers in operation. The ER engines do not have to be separately tested as their maximum thrust rating has already been certificated for the 747-400.

Both versions then have to undergo special fire protection tests. With the aid of smoke generators, the cabin bays are progressively filled with smoke until visibility is restricted to 45cm. On the one hand the smoke alarms, which have a new design, have to detect the slightest trace of smoke, while on the other hand the smoke, which is soon thick enough to cut, must not get into the cockpit or passenger cabin via the air-conditioning system. Fire fighting is actually impeded by the way the air-conditioning works impedes, as Program Manager Kurt Kraft explains. "Our fire extinguishing system requires a quite specific amount of halon to combat fires. So we try to achieve that volume of halon as a permanent concentration. However, the pressurised cabin is continuously vented via a valve at the rear so that fresh air can stream in. The trick lies in not blowing the halon out of the plane along with the extracted air.”

For certification, full fire fighting must be demonstrated to the Federal Aviation Administration, while in addition the aircraft executes an emergency descent manoeuvre from the cruise altitude to 15,000ft (approx. 4,600m). Because of the precise calibration of the pressure relief valve that is necessary in advance, Kurt Kraft, with the rationality of an experienced engineer, calls this part of the testing a "particularly demanding task”.

With all the tension around, are any of the trials actually boring? Art Fanning does not utter the word, but the painstaking measurement of the electromagnetic compatibility of avionics and electronic on-board entertainment system come the closest, in his view.

The second test aircraft "RF-002” with newly designed "Signature” cabin furnishings is primarily being used for these tests. This entails checking with the utmost precision with mobile antennae in the cabin whether the on-board electronics is producing any interference. A never-ending series of combinations of equipment from a wide range of manufacturers, some of which are only offered as an option, has to be considered here. "We have to be completely sure that it will not cause any interference later on,” explains the Flight Test Manager. However, the two experts are not prepared to disclose their trade secrets of how to improve equipment shielding if required.

Practical on board comfort is also tested. Does the air conditioning cause draughts, are all the announcements intelligible? Does the heating work as it is supposed to? How loud is it in the cabin at different stages of a flight? Finally, individual compartments of the cabin are occupied by test passengers in order to test the on-board entertainment systems, toilets and kitchens under realistic conditions.

During the interview with FLUG REVUE, at which point 70 out of 76 days of the testing programme have been completed, all that is left is to run through the full gamut of tests once more, this time in front of the official FAA inspectors – the final step before final certification of the Boeing 747-400ER and the 747-400ERF freighter. "We have stuck to the test schedule that we announced a year ago,” says Kurt Kraft proudly. "Our jumbo will be ready on time.”

From page 6 of FLUG REVUE 12/2002

Last updated 4 November 2002

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