If all engines fail in a Boeing, how heavy are the flight controls?

Question

Consider a large airliner with no Fly-By-Wire controls, such as a Boeing 747 or 767. All the engines have failed, the Ram Air Turbine is deployed, and the plane is gliding.

How heavy are the flight controls in this situation?

What about Boeing 777 and 787, which are Fly-By-Wire aircrafts?

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EDIT: Regarding the question "how heavy", I would like to get a number, or if that's not possible, a description from a pilot whether it is physically exhausting to hold the controls.

Answer 1

A 747 doesn't have a ram air turbine (RAT), but it has an APU that can power the hydraulics. If that can't be started, a 747 can still function using the hydraulic pressure from each engine's windmilling. Computer based training video here.

Trivia: if all hydraulics fail, but engines are running, a 747 is a dead stick, but can be steered using the engines. A complete loss of engines and hydraulics is never in the emergency procedures of a heavy airliner.

In a 767, assuming the APU also won't start, the RAT will provide hydraulic pressure. Controls will be heavier (but not impossible) the slower the plane is.

^{Source: 767 QRH, 13.14}

^{767 RAT speed below 20,000 feet is >200 IAS, any slower, hydraulic pressure will be lower.}

A 787's artificial feel system (Elevator Variable Feel) will revert to direct mode, feel will be based on flap position, and during approach feel forces will be reduced to make the landing easier.

^{Source: 787 FCOM, 9.20.12}

Trivia: a 787's RAT can supply 5000 psi of hydraulic pressure.

In a 737, the crew will use manual reversion mode, for roll and pitch it's control tabs, like an MD-80, heavy but not impossible.

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For all types, it will be almost impossible to find data on the exact amount of force needed. Also for all types, if no APU or RAT, windmilling can still provide some hydraulic pressure.

Same for the pilot feedback, as very few flew all-engine-out heavies. And adrenaline rush would almost certainly skew their post-flight judgement.

Answer 2

To get an idea of the maximum forces, look at the certification limits. FAR 23.143 gives the limits for GA aircraft and FAR 25.143 those for larger aircraft including airliners. If these forces would be exceeded, even when something has failed, the aircraft would not be certified.

The short-term limits in pitch are:

• 60 lbs for stick-controlled aircraft (only covered in FAR 23),
• 50 lbs when only one hand is available for control, and
• 75 lbs when both hands can be used for control.

The short-term limits in roll are:

• 30 lbs for stick-controlled aircraft (only covered in FAR 23),
• 25 lbs when only one hand is available for control, and
• 50 lbs when both hands can be used for control.

The maximum force limits for prolonged application are much lower again.