19 February 2016, ©. Leeham Co: Last week I described how Mitsubishi Aircraft Corporation (MAC) issued a press release on Christmas day communicating MAC would be doing structural reinforcements on their test airframes before continuing flight testing. MAC was perhaps overzealous when informing the world that they would do minor reinforcements to two ribs and a few stub spars in order to pass Ultimate strength tests for the aircraft.
I rightfully thought this is the Japanese culture at play; there must not be a big problem behind it.
There was one more area of that press release that intrigued me. Here what it said: “The first flight and the subsequent flight tests have confirmed the basic characteristics to be satisfactory. However, we also have recognized several issues as we attempt to accelerate our development.”
Time to decrypt this as well and compare to what has become standard industry practice.
First test flights
The released sentences smelled of troubled flights. At my January meeting with MAC’s Marketing Director, Hideyuki Kamiya, I asked him what was the meaning of this part. He answered the test flights were normal, nothing special was discovered and they were satisfactory. I told him “once again your language is other than your colleagues.” Instead of dwelling on MAC’s language, let’s look at what MAC’s colleague OEM’s typically say.
“It was a joy to fly, it really flies well,” “The aircraft flew beautifully,” “It was a perfect flight, we only had one small problem,” and so on. Behind those upbeat expressions after the first flights, one cannot really read anything else than that the flight was not interrupted for some reason. I know that during several of these flights, there were real problems found, some of which merit to stop the flight and going home in emergency mode.
Today’s aircraft are extremely well analyzed and tested aerodynamically before first flights. Detecting major flaws in how the aircraft flies as an aircraft is less probable. On the other hand, the systems side and especially avionics and flight controls have grown in complexity to a level that makes test crews really nervous.
A modern airliner has between 10 million to 15 million lines of software code in its systems. The likelihood of some of that software going into wrong modes or outright stopping to work is high. One of the calmest first flights had the air-conditioning flying to New York when the aircraft was circling the Pyrenees. Another had the whole electrical system shutting down for a while.
Two very critical system areas are the Fly-By-Wire (FBW) and the computerized engine control, the FADEC. For first flights, the FBW is virtually taken out of the equation. Civil Airliners have to be naturally stable also when equipped with FBW. This means the aircraft will sort itself out of troubled flight situations without FBW intelligence and with minor pilot input.
Consequently the FBW is set to work as if the contact with the rudders was made with steel wires, the so-called direct mode. It means a minimum of involvement from the complex FBW software and therefore a minimum chance that things could go wrong. It’s also the preferred mode from the test pilot’s viewpoint for first flights. First flights are when you explore the base behaviour of the aircraft.
You don’t want an FBW or autopilot which adds or subtracts to or even masks the base aircraft’s quirks during such flights. It’s the only way to add the human input to all the data when the flight gets evaluated after flight. FBW intelligence is added much later in the flight test programs.
For the engine’s FADEC, it’s even worse. You don’t fly it first on the test aircraft. The engines are test flown on the engine manufacturers own four engine test aircraft, a Boeing 747 or loaned A340/A380. One of the four engines is replaced or there is a special fifth pylon added for the test engine. Only with such a setup can a complete FADEC run-away be handled. You stop the engine by shutting its fuel supply in worst case and fly home on the remaining engines.
Only when the engine and its computerized control are rock solid is it allowed to fly on the airliner test aircraft. And even then it can go wrong, as it did for the half military-half civil Airbus A400M. A new FADEC software version which was tested to be solid did not install correctly on the target aircraft. The consequences in the ensuing test flight were fatal.
I would say the least worry I would have as a test pilot today would be how the base aircraft flies. At least as long as one follows the mother’s advice “Fly slow and low.” Once one get to high flight levels and transonic speed it gets more interesting.