In a 777, what would happen if the auto-pilot and the altitude hold failed at the same time?

Question

I know nothing about a Boeing 777 flight controls.

In general, I know that the speed of an aircraft is controlled by the throttle on the center console. I have never heard of an automatic speed control. I believe the pilot manually adjusts the speed. I have heard of altitude hold control. You set it and the system keeps the aircraft at the same altitude. Is this independent of the auto-pilot? And of course there's the auto-pilot. It makes the aircraft go in a direction you want it to go.

If the pilot does not intervene, what would happen if the auto-pilot and the altitude hold failed at the same time?

• Does the aircraft climb or dive? Dose it oscillate up and down?
• Does the aircraft roll over?
• Does the aircraft turn?
• What about speed?
• If it continues in some fashion, what is the effect of the fuel consumption in the different tanks? Is it automatically controlled? What happens if that control fails?

Answer 1

Regardless of whether it's a 777 or any other airplane where the altitude is being held by the autopilot and the altitude hold function stops, whether because of general autopilot failure, just altitude hold failure, or the altitude hold function is inadvertently or purposely switched off; the airplane will seek whatever pitch attitude it is trimmed for.

For example, using the 747-100 and -200 autopilot setup, let's say you are in cruise at 35,000 ft, the autopilot is navigating and altitude hold is on. Further, let's say we're beyond 1990, and many operators of those aircraft had disabled autothrottles because they were a maintenance hog. Thus the airplane is maintaining whatever speed has resulted from where you have positioned the power levers. The altitude hold function is maintaining altitude by using the elevator. You can look down at an indicator on the center console between the pilots to see the degree of up or down elevator being used.

As the flight progresses and fuel is burned, the c.g. moves forward and the altitude hold function adds more up elevator to maintain altitude. When a certain amount of up elevator amount is reached, the system automatically retrims so that no up elevator force is required. You hear and see it happen because the large trim wheel on the right side of the center console rotates.

If you're a good captain or a good flight engineer, you can monitor the amount of up elevator force being held and trim it out manually before it reaches enough to trigger the automatic retrim, and save the company a little fuel money (or if you're JFK to Tel Aviv in a -100, possibly avoid a fuel stop in Athens).

Now let's say that you reach up and disengage the altitude hold sometime between retrimmings. The airplane will nose down, and actually quite abruptly if it's been awhile since it last retrimmed. So abruptly that normal practice was to always have the yoke in hand before disengaging the autopilot.

Let's say that you that you let the condition persist. The nose is down, so the airplane would descend, of course. The speed would oscillate during the descent. If there was a severe nose down trim ... well, you wouldn't want to go there.

For a humorous account (at least meant to be humorous) of the altitude hold function being inadvertently disengaged, go to http://terryliittschwager.com/hajj4contest.php.

Now for the case of total autopilot failure, I defer to the other answer already here when I posted this. Suffice it to say that the pilots would have to take control in short order to avoid an extremely serious condition, very possibly the loss of the aircraft. You can go to http://community.seattletimes.nwsource.com/archive/?date=19911219&slug=1323954 for information on an incident that involves such. The interesting thing is this airplane had started to do this a number of times (including once to me coming out of Hong Kong), but in previous instances the pilots quickly recognized that the autopilot had failed and had taken control before things got serious.

Following the incident described, the airplane was repaired and reentered service, and we called her Christine.

Answer 2

• No, speed is not controlled by "throttle" on the center console¹. Due to the way aircraft longitudinal stability works, the elevators (controlled by control columns (yoke)) control speed and thrust or power² levers control climb rate.
• Automatic speed control, "autothrottle" (A/T) is a cockpit automation mode that adjusts thrust to maintain speed. Since thrust does not control speed, it relies on someone operating the elevators to maintain flight path or at least attitude. This would normally be autopilot³.
• Altitude hold is one of the modes of "autopilot" (A/P). Autopilot controls both pitch and roll. Via pitch it can control vertical speed or altitude and via roll it can control direction. The term autopilot may be used including or excluding A/T.
• At altitude it becomes increasingly difficult to fly by hand. To allow lower separation, aircraft are only allowed above FL290 if they are equipped with working altitude hold and pilots would normally use it.
• That does not mean the pilots are not watching the instruments. In case the autopilot fails, they would resume flying manually.
• If the pilots did not assume control, than the aircraft would start a Phugoid oscillation. It would continue straight, but since the lateral stability is rather weak⁴ if turbulence caused it to bank to the side, it could overbank and enter Graveyard spiral.
• Normally engines only take fuel from their side⁵. But both engines are normally consuming the fuel at the same rate and the initial amount is the same on both sides, so they would both run out quickly after each other.

¹_{Except if you engage the autopilot vertical speed or altitude hod mode or in Airbus with it's normal law which automatically retrims the aircraft for current speed to maintain flight path which is similar automation but built into the system and never disengaged except in case of failure.}

²_{"Throttle" refers to the flap restricting airflow to the engine, which is specific to spark-ignition reciprocating engines. Compression-ignition (diesel) engines and turbines are controlled by directly adjusting the fuel flow, don't have throttle and thus the levers are not usually called "throttle" in them. They are called "power" levers for propeller engines (turboprop and diesel) and "thrust" levers for jet engines, reflecting the difference in the engine behaviour with increasing speed.}

³_{While A/P and A/T can always be engaged independently, in most aircraft it is recommended to always used both or neither. Exception is again Airbus where A/T without A/P makes sense.}

⁴_{Making the aircraft laterally stable is possible, but unfortunately it aggravates the Dutch roll mode. So usually the lateral stability is limited and will only return the aircraft to level from small disturbances.}

⁵_{I am not sure whether on B777 the centre tank feeds the engines directly or transfers to the wing tanks, but the result is not much different.}