A closer look at the DA42 Variable Elevator Backstop
A common issue with T-shaped tails is to get less airflow at high pitch because elevator’s airflow is obstructed by the rest of the fuselage, possibly leading to a tail stall. I’m not sure if this is the reason why the DA42 TwinStar elevator is so large but this extra-size makes it very effective. It is so effective that its use has to be limited in flight to meet certification standards. Feeling confused ? Let’s have a closer look.
The DA42’s BEFORE START CHECK includes checking the “Variable Elevator Backstop”. This check is done as follow:
- Set flaps to landing positions
- Hold the stick to the full aft position
- Push throttles forward (engines not running)
- Check for stick limit decreasing
- Retard throttles
- Check for stick limit increasing
When the throttles get pushed, an electrically activated actuator actually restrains the stick freedom of movement in the aft direction. If the stick is held back, the actuator simply pushes it forward. I felt it on the plane and this is also a feature of the simulator. When the conditions leading to the limitation (flaps in landing position, landing gear down and throttles above 20%) are met, the maximal possible elevator deflection is 13°, instead of the normal 15.5°. Concretely, it is possible to use the 15.5° deflection on flare, because power is reduced, but not in case of go-around because full power is set. The actuator takes a couple of seconds to push the stick forward and it is a really strange feeling for the pilot during the test.
I found the reason behind this surprising system in this DA42 certification document from the New-Zealand Civil Aviation Authority: “CRI D-02 Issue 4 19.4.04 – SPECIAL CONDITION – Variable Elevator Stop. The aircraft has an electrically operated actuator which limits elevator up travel above a certain engine power setting to ensure compliance with the stall requirements of 23.201 and 23.203.”
The FAA certification standards regarding stalling characteristics are in FAR CS 23.201 and 23.203. The FARs are not always easy to read and understand and these two paragraphs are no exception. They define what is a stall, how the aircraft shall react and how testing must be conducted: “After the airplane has stalled, as defined in Sec. 23.201(b), it must be possible to regain wings level flight by normal use of the flight controls, but without increasing power and without
- Excessive loss of altitude;
- Undue pitchup;
- Uncontrollable tendency to spin;
- Exceeding a bank angle of 60 degrees in the original direction of the turn or 30 degrees in the opposite direction in the case of turning flight stalls;
- Exceeding a bank angle of 90 degrees in the original direction of the turn or 60 degrees in the opposite direction in the case of accelerated turning stalls; and
- Exceeding the maximum permissible speed or allowable limit load factor.“
The definition of stall according to 23.201 is:
- An uncontrollable downward pitching motion of the airplane;
- A downward pitching motion of the airplane that results from the activation of a stall avoidance device (for example, stick pusher)
- The control reaching the stop.
The DA42 has no stick pusher and no strong tendency to pitch downward, so the stick limit is an important factor. As the final approach speed is rather low (approximately 80 KIAS - click here to read more about the DA42 V-Speeds) the elevator has to be performant. Finding a trade-off between sufficient efficiency at low speed and acceptable stall characteristics at full deflection with power and full flaps was apparently not possible so the Diamond design team had to limit the full deflection for certain phases of flight.
As any system, the variable elevator backstop can fail. This is reported on the G1000 if the system detects that the flaps and throttle conditions do not correspond with the expected actuator position. This can happen on ground before take-off if the actuator is in the limiting position, or on short final or during go-around when full power is applied. The next step in a go-around is to raise flaps to approach position so the time flown with power and landing flaps is short… but who wants a hard to recover stall close to the ground ? If the “STICK LIMIT” warning pops-up, two cases must be distinguised (quoting the aircraft manual):
- The variable elevator backstop is inoperative. In case of stalling with “power-on” the handling qualities and stall- characteristics are degraded significantly. Do not stall the airplane in any configuration.
- The variable elevator backstop is active all the time, reducing the maximum elevator “pull”-deflection. This results in
reduced elevator capacity. In this case it is important not to reduce airspeed below required minimum vREF during the approach for landing, especially at loading conditions with forward locations of the center of gravity.
This kind of detail, a small system with a short check-list, can easily go un-noticed. The “RTFM” tip from Matthew about flying the SR-22 is valid here again. Know your aircraft inside-out or a bad surprise can quickly become an emergency if not worst…
