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Envelope Protection and Twins

Wouldn’t the envelope protection bring most value in twins?
Why it’s not done? Is it a technical reason behind or just economical?

LRTC, LRPV, LFPN

How would it help with a twin more than a single?

EGTK Oxford

The potential for Vmc to be below Vs resulting in asymmetric yaw close to the stall with a resulting loss of control. It was the Twinkie Vmc demonstrations that led to the introduction of Vsse (safe single engine speed). Even quite recently Vmc demos are claiming the lives of students and instructors. In the same vein stall recovery exercises for issuing or renewing multi ratings, you initiate recovery at first indication of the stall, not allowing a stall to be fully developed.

Oxford (EGTK), United Kingdom

Well yes of course but I can’t see how Garmin or Avidyne type envelope protection can help you. They typically work by effectively stick pushing to prevent a stall. How could envelope protection help you in a light twin if you lose an engine below Vmca? Without being able to control throttles I can’t see how it could recover.

EGTK Oxford

Some twins have a Vx which is below Vmc, but generally speaking operational speeds will be above Vmc. Would agree that without rudder boost, split torque sensors and auto feather and throttle, not sure whether envelop protection would cope with an OEI twin. Arguably the safe envelop panic button would require bringing both throttles to idle, and it would place the aircraft in a glide at Vyse or Vy, which is reasonably close to Vmd, or a glide at the equivalent of Va.

I think the OP may have posted implying a twin has more of a possibility to encounter scenarios where departure from controlled flight was a possibility. Not sure I would agree, given higher wing loading and usually quite solid stability characteristics in a twin. A check of the NTSB database may show whether twins have more in flight break ups, or loss of control in IMC, than high performance singles. My assumption is possibly not, despite the likelihood of spending more time in IMC.

Oxford (EGTK), United Kingdom

I agree and would think they are less likely to depart other than when slow and OEI.

EGTK Oxford

Not sure if I understand the original question and if it relates to OEI or in general.

But in a OEI scenario, envelope protection could potentially help with:

1) Pitch forward to maintain the greater of Vmca and Vs (even if flies you into the ground)
2) Pitch up to maintain Vyse until safe altitude
3) Bank into the live engine to optimise climb rate and minimise Vmca (certified Vmca speed assumes bank angle and actual Vmca is higher without)
4) Apply rudder to maintain heading / eliminate side slip / maximise performance, although you don’t want this masking the failed engine before identifying it

But this assumes envelope protection knows there is an engine failure. Presumably you would identify and secure the failed engine, then press the button.

If below Vmca already I agree it might be too late, envelope protection could only pitch down to accelerate, if unable to retard the throttles.

A covert Boeing v. Airbus thread :)?

How much of the above does Airbus do when OEI (assuming normal law)? Presumably all of this plus more.

All this begs the question how much automation do we want in private aviation, i suppose.

Although i personally enjoy flying an aircraft and trying to understand as much as i can how it behaves and why, and act accordingly, from safety point of view it may be a lot better to have automation take over. If that happens i would stop flying, and i somehow think lots of other would do too. Imagine riding a Harley with your hands behind your back and be taken for a ride..

Commercial aviation may be a different matter though, where we are probably on the path of complete automation, and accepting the fact that -when it fails, very rarely, – clueless operators will fly themselves, and their passengers, into the ground or sea. In future the operators may not even be on board.

Private field, Mallorca, Spain
22 Posts
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