September 6, 2019, ©. Leeham News: In our series about classical flight controls (“fly by steel wire”) and Fly-By-Wire (FBW or “fly by electrical wire”) we discussed the flight control laws which are implemented with classical flight controls compared with the Embraer E-Jet and Airbus A320 FBW systems last week.
Now we describe alternative FBW approaches, analyzing Boeing’s 777/787 system and Airbus’ A220 system.
The Boeing FBW system philosophy was developed with the 777, which entered service 1994. Airbus had presented their FBW philosophy with the A320 seven years earlier.
Boeing chose to not follow Airbus in how the FBW system interprets the Pilot’s intentions. It wanted a system which should feel like a classical mechanical system but with additional comfort and safety functions realized in its feedback FBW architecture, Figure 1.
The philosophy is the pilot is always in control and he can overload or stall the aircraft if he needs to in order to save the aircraft from a hazard. The FBW warns him, however, when he’s approaching the limits of the aircraft. The trim stops when he approaches very low or high speeds, so he needs to continuously push or pull the Yoke for speeds outside the normal. In addition, his controls get successively heavier as he approaches the aircraft’s limits. Finally, a stick shaker activates when approaching a stall.
The system does not auto-trim, the Pilot has to trim the aircraft to balance the aircraft for a certain speed. This is called the aircraft is speed stable (if the speed drops the aircraft dips the nose and regains speed). In general, the philosophy is the aircraft shall behave like the classical aircraft the pilot trained on to get his basic flying skills.
The 787 system has the same behavior as the 777 and by it, the aircraft can share Pilot type rating. I’ve been told the 787 introduces hard stall protection in pitch when in landing configuration. If so, this makes sense as when close to the ground hard protection from stalling the aircraft makes sense.
When the Bombardier CSeries team designed the FBW for what today is called Airbus A220, they took the best ideas from Airbus and Boeing and put them together in their system. The FBW feels like a Boeing system in normal flight, with the Pilot needing to trim the aircraft for a certain speed. This trains the Pilot for an emergency where the FBW would fall back to direct mode.
“The A220 behaves like the Cessna 152 the pilot trained on. This put’s his muscle memory in the right mode if he gets in trouble and has to rely on direct FBW mode” said Chuck Ellis, the CSeries chief test pilot when I test flew the CS300 in Wichita, KS.
Different to Boeing, the CSeries team combined the “classical” FBW feel with envelope protections like the Airbus FBW. Here the team chose a clever principle, however.
Boeing’s argument for a Pilot in control is he shall be able to overload the aircraft if it’s needed to avoid a hazard. An Airbus FBW will stop the Pilot from overstepping the envelope limits of the aircraft, be it load factor, stall speed or over-speed.
The A220 allows the Pilot to overload the aircraft when in trouble, but only when he pulls the stick past an extra force limit, called a “Soft stop”. Then he gets a load factor in pitch above Limit load (the maximum load allowed in normal flying) but there is a “Hard limit” stopping him to pass Ultimate load (the strength limit of the aircraft at 150% of Limit load). By it, the team argued they leave the Pilot in control but the FBW stops him from breaking up the aircraft. If a Pilot pulls the stick past the Soft stop the aircraft needs a structural inspection before cleared to fly again.
When test flying the CSeries the two regions of the stick felt very natural. The normal region is for normal flying and the “hard to pull” region is for serious trouble. As I haven’t flown the Boeing FBW I can’t say what I like best of the different systems. Between the Airbus classic FBW and the A220 system, I feel the latter is the better approach.
The auto-trim of the Airbus FBW is very convenient and many FBW systems implement it (for example the Daussalt Falcon Biz jets). But it trains the muscle memory away from Direct mode flying of the aircraft. A trim for speed FBW is more work, but it retains the flying feel of a normal aircraft in the Pilot’s subconscious muscle memory.
In the next Corner, we take a look at the E-Jets second-generation FBW before we dig deeper into the safety aspects of flight control systems by discussing helper systems like the 737 MCAS system.