After sitting idle for 100 days, the CSeries fleet returned to the air with a 30min flight by FTV-2 on 7 September and another 3h test mission on 9 September.

Bombardier still has much to prove in the air and around airline boardroom tables with the CSeries programme, but the return to flight following a 29 May engine failure and minor component redesign marked a key step in getting back on track.

“It was a great day for the CSeries,” a breathless, Bombardier vice-president Rob Dewar said in a company video online shortly after the 7 September flight, just before the CSeries programme manager took off for Zurich to meet launch customer Swiss International Air Lines.

“So after, I would say, a very productive summer finding a solution, and we’re good to go,” Dewar says. “That’s great news.”

Dewar had more good news to share with the Lufthansa subsidiary, which placed an order for 30 CSeries aircraft in 2009.

As the CSeries test fleet sat on the ground all summer awaiting a Pratt & Whitney engine modification, Bombardier’s software team and suppliers stayed busy. The programme used the downtime to install the Build 3 version of the flighteck software, enabling the test team to “soon” activate a critical feature of the aircraft’s fly-by-wire control system.

One of the biggest mysteries surrounding the programme since FTV-1 achieved first flight on 16 September last year has been the status of the fly-by-wire system. Since March, company officials have simply declined when asked to provide updates about its status.

Bombardier now confirms that the programme has still not activated the normal law of the fly-by-wire system in flight, despite accumulating more than 300 flight hours in a year. In normal law, the fly-by-wire system activates flight envelope protections that prevent the aircraft from stalling or over-speeding. It is the fly-by-wire mode of most interest to certification authorities, as the aircraft will almost always operate in normal law during commercial service.

Instead, the CSeries test fleet has operated to date in flight in a degraded mode called direct law. In this mode, the software still interprets the pilot’s control inputs and sends commands to the flight control surfaces, but it does not provide flight envelope protections.

Although necessary to achieve many test points, such as the aircraft’s stall recovery profile, direct law often yields to normal law operations as early as possible in a flight test programme. Airbus activated normal law on the A350-900, for example, on that aircraft’s first flight in June 2013.

Last February, then-Bombardier Aerospace chief Guy Hachey said that the Build 3 software would be installed in the flight test aircraft the following month. Asked for a status update in March, Hachey indicated no further updates would be forthcoming during the flight test programme.

Bombardier officials, however, did say that the Build 3 software had been installed in an elaborate ground-based simulator called the integrated systems test and certification rig. In the months since March, the programme’s fly-by-wire team has fully tested the software on the ground, a Bombardier spokeswoman says.

The Build 3 software also was loaded into the FTV-2 flight control computer during the 100-day hiatus caused by the 29 May engine failure, she says. Normal law was not activated during the first two flight tests, but it will come online in flight shortly, she adds.

Bombardier has estimated that the CSeries must complete at least 2,400h of flight testing to complete all certification requirements. At the time of the grounding on 29 May, the progrgamme had completed slightly more than 300.

“The next steps are to get the other FTVs back in the air and let’s really get the flight test going,” Dewar says.

FTV-4 is scheduled to be the next test aircraft to resume flying. FTV-3 will return to later flight later “this fall”, Dewar says, following return to flight of the damaged FTV-1.

On 29 May, P&W test crews were conducting stationary maintenance checks on the left-hand PW1500G, a geared turbofan engine. A problem in the lubrication system triggered the failure of a stage in the low-pressure turbine section. Small pieces of the engine escaped containment and punctured the left-side, carbonfibre wing of FTV-1.

As P&W worked to redesign the oil system to prevent the lubrication problem from happening again, Bombardier’s structural engineers got early practice repairing the carbonfibre skin of the CSeries, Dewar says.

The CSeries is the second aircraft – following the Learjet 85 – in Bombardier’s portfolio to use carbonfibre wing panels manufactured using a resin transfer infusion process developed by the company’s Short Brothers division in Belfast, Northern Ireland.

Bombardier did not expect to face a serious carbonfibre repair job until after the CSeries entered service. The test incident offered a rare opportunity for the company to come to the grips with a real-world damage problem during the flight test stage.

“The repairs are going very well,” Dewar says in a separate video released by the company two days before the return to flight of FTV-2. “They are nearing completion as we speak. It was also a good opportunity for our team from Belfast to repair a carbonfibre wing.”

The last aircraft in the flight test programme for the 110-seat, CS100 variant of the CSeries family is FTV-5, which will be the first test aircraft to be equipped with a representative passenger cabin.

Bombardier announced in March 2013 – 18 months ago – that FTV-5 had entered final assembly. In July 2012, Dewar described a plan to roll out the five test vehicles at a rate of one per month following first flight of FTV-1. Twelve months later, the cabin demonstrator remains inside Bombardier’s final assembly hangar in Mirabel, Canada. FTV-5 is now scheduled to join the flight test programme by the end of this year, Dewar says.

The combined test fleet must average more than 130 flight hours per month to meet Bombardier’s goal to deliver CS100 to an undisclosed launch operator by the end of next year.

Inducting all five aircraft in the flight test programme will help ease the pressure, but the fleet must resist a year-long tendency to make progress very slowly. In addition to the 100-day grounding that began on 29 May, there was a 27-day flying hiatus that started within two weeks of FTV-1 first flight last year.

It then took Bombardier four months to complete more than 100 flight test hours, a period stretching from September to the end of January. As FTV-3 and FTV-4 entered the test fleet in March and May, flight hours doubled over the next three months. The fleet now will have to maintain that pace and grow significantly to stay on track with Bombardier’s current schedule.

The engine problem required 100 days to fix, but Bombardier and P&W discussed redesign options for several weeks. In the end, the new design should prevent a recurrence, but without sacrificing performance. The PW1500G still is on track to meet the programme’s thrust, fuel burn and emission goals, Dewar says.

“Of course, we never like to have these events,” Dewar says, “but that’s why we do flight test.”