Why don’t large aircraft (larger than ~737-size) have even partial manual-reversion capability?

Question

Essentially all large aircraft¹ have flight control surfaces powered, either directly or indirectly, by hydraulic actuators, as do most medium-sized and some small aircraft. Small- to medium-sized aircraft with hydraulically-actuated flight controls essentially always have some form of manual reversion capability, where the primary flight control surfaces (always the elevator and ailerons, and sometimes the rudder as well) are operated manually by the pilots, usually by control cables attached to the pilots’ yokes and pulling (depending on the aircraft) either on the control surfaces directly or on servo tabs which then generate aerodynamic forces which move the control surfaces. This allows the flight control surfaces to remain operational in the event of a total failure of the aircraft’s hydraulic systems, albeit requiring the pilots to exert a (sometimes considerably) greater force on the controls to operate them.

Large aircraft, on the other hand, generally have no manual-reversion capability at all. This is ostensibly due to the greater aerodynamic forces acting on their flight control surfaces at cruising speeds, which would require superhuman strength to completely overcome (despite the fact that, before hydraulic flight control boosting came into wide use, many, many large and fast aircraft were successfully flown with purely manual controls through the use of a number of force-reducing tricks). However, in a no-hydraulics situation, having any manual control capability at all, even with a significantly- to considerably-reduced control authority and range of movement compared with hydraulically-boosted operation, would be an extremely-useful adjunct to throttle pitch control and steering (which – especially throttle steering – are difficult, sluggish, clunky, and only really usable for coarse altitude and flightpath adjustments). So why don’t large aircraft have at least partial manual-reversion capability, despite even a little flight control authority being far, far better than nothing at all?

---

¹ I'm using "large" here to mean "larger than a 737 MAX 10".

Answer 1

For aircraft that don't have manual control capability, the required redundancy is built into the system design with redundant hydraulic actuators powered by redundant hydraulic systems.

You can't look at it and say that some worst case is theoretically possible and you have to design for that. The industry uses a failure analysis and risk probability approach for failures, with 4 main risk categories; Minor (not too big a deal, just extra workload - probability of 1:1000), Major (Significant increase in workload or difficulty for crew - 1:10,000), Hazardous (Someone gets hurt - 1:10,000,000), and Catastrophic (loss of the airframe) (1:1000,000,000).

So for Catastrophic events, the probability of a single component failure that causes loss of an airframe has to be better than one in a billion, and if not you have to add a backup to get it below that. One in a billion is pretty unlikely, but not impossible, and outlier failures happen from time to time, and sometimes it's because the system safety analysis itself was flawed (that happens in a lot of cases).

What it means is that if the fully hydraulic controls are designed with sufficient damage tolerance and redundancy to keep potential failures within that risk profile, no further backup is required, and such additional equipment is effectively considered "ballast" from a design for risk perspective.

It's a cold hearted numbers game on the surface, but you have to create some kind of arbitrary mathematical model to go from or none of it would work because airplanes would have to be overdesigned to death to cater to every possible eventuality. Great fodder for trial lawyers though.

Answer 2

Adding in manual control mechanics for a large aircraft has serious drawbacks:

• Weight: For a large aircraft, the weight of pulleys, cables, etc., will add up quickly. This costs money in fuel and by reducing the useful load of the aircraft.
• Complexity: The manual system would have to be routed through the entire aircraft, including passing through pressure bulkheads and other structure. This takes a lot of time to design, install, and maintain. While there are certainly ways to deal with the high forces involved, that just adds more complexity.
• Vulnerability: There have been multiple accidents in which manual control systems were improperly set up or jammed. The manual system would have to be designed so that fly-by-wire system would still work even if the manual system develops an issue. If an event is catastrophic enough to cripple all of the hydraulic/electrical systems, there's a good chance a manual system would be disabled as well.

The alternative is to design a hydraulic system that can match or exceed the reliability of a manual system. Of course this is not without its own complexities and issues, but virtually all modern airliners have gone this route. The weight and complexity of a manual system are not worth the remote probability that such a system would be both necessary and useful.

Answer 3

Boeing aircraft, such as the B737 can be flown with manual reversion for ailerons and elevator following complete hydraulic failure/loss. Other large aircraft also can be flown using manual reversion (e.g. MD 80, etc.).

Plenty of references available. Here is some info from page 16 of an NTSB accident report for a B737: link

From page 16 of the accident report identified by the link above: