Six months beyond first flight, Bombardier has still not activated the essential feature of the CSeries fly-by-wire flight control system, but company officials insist there is no fundamental design or engineering problem.
“I know there’s a lot of people saying you’re not in normal mode, so you’re in big trouble,” says Guy Hachey, president of Bombardier Aerospace. But “it’s not necessary at this stage in the testing.”
Fly-by-wire technology replaces mechanical linkages physically connecting the pilot’s control stick to pulleys or hydraulic actuators that move the flight control surfaces.
The aileron, rudder, elevators and spoilers are instead activated by signals from a computer, which interprets inputs from the pilot’s sidestick.
This digital system helps reduce weight and maintenance costs of the mechanical linkages, and also introduces safety features. For example, a fly-by-wire system prevents the pilot from over-rolling the wings or flying too fast.
But these envelope protections are activated only when the fly-by-wire system is operated in “normal” mode.
Bombardier introduced a fly-by-wire-controlled rudder on the CRJ1000, but the CSeries is the company’s first attempt to certificate an aircraft with a system that moves all the flight control surfaces.
A glitch in the fly-by-wire software on the CRJ1000 rudder delayed the certification programme for nine months, so Bombardier has always said it will move cautiously with the CSeries system that comes from the same supplier – Parker Aerospace.
So far, the three CSeries flight test aircraft have operated only in a degraded “direct” mode, in which no envelope protections are activated.
Bombardier officials have previously said that a new software block enabling the normal mode would be ready for the flight test aircraft by March.
However, Hachey and Philippe Poutissou, Bombardier Commercial Airplanes vice-president of marketing, insisted in a company-hosted investor day on 20 March that the lack of testing in normal mode so far does not spell new trouble for the programme.
“It’s no big deal,” Hachey says.
The normal mode software block is still being tested on the CSeries integrate systems test and certification rig (ISTCR) in Mirabel, Canada, Poutissou says.
“When the time will be right to test our normal mode we will test on our aircraft,” he says.
Flight testing so far has focused on clearing the envelope, which includes stall testing. The next round of tests will focus on testing the flutter characteristics of the structure, Hachey says.
In such tests, the CSeries aircraft has to be flown in direct mode anyway, or the envelope protections enabled by the normal mode would prevent the aircraft from stalling or stressing the aircraft to gather flutter test points, he adds.
It is still unusual for an aircraft manufacturer to wait so long to begin testing such an essential feature of the flight control system. Airbus, for example, activated the normal mode of the A350-900 in the first flight last June.
But Bombardier executives say they are confident that the CSeries will be delivered in the second half of 2015. After a painfully slow start, CSeries flight testing is starting to build momentum. The rate of testing will double from February to March, Hachey says, and the pace should continue to quicken in April.