# Why is stabilizer trim often controlled by electric motor while other surfaces are mostly hydraulically controlled?

What is the rationale behind the use of electrical motors instead of Hydraulic actuation for the stabilizer which I have seen in most aircraft like Boeing 787.

• Most trim is either manual or electric. Trim changes do not have to be quick, but should be precise. Control surfaces like the elevator must be able to move quickly, so big aircraft use hydraulics for that. – Peter Kämpf May 29 '14 at 17:07

Weight, cost, reliability and maintenance.

Safety

In terms of safety several aspects can be highlighted: As far as power source redundancy is concerned the number is increased from 3 to 4 since 2 electrical systems replace 1 hydraulic system. Furthermore an additional margin of safety results from the introduction of the hydraulic/electric dissimilarity in the power sources: This provides further protection against common failures, such as maintenance errors, which may affect all the hydraulic systems, whatever their number. Moreover the electrical power provides flexibility in routing, resulting in an easier segregation of power distribution routes against engine burst and other “particular risks”, and an isolation and reconfiguration capability that hydraulic systems cannot offer.

The reduction in the total number of hydraulic components results in improvements of the MTBF and dispatch reliability, by elimination of potential leakage sources.

The elimination of the generation and distribution components associated to one of the hydraulic systems (pump, reservoir, filters, plumbing…) and the replacement of the associated hydraulic actuators by electrically powered actuators results in weight savings.

• Could the plane (DC10?) that lost all hydraulics perhaps have landed better if electric trim tabs had been available and functional? My impression is that it landed very rough because there was no way to stabilize the fugoid oscillations using only thrust controls, but I would think being able to adjust trim might have made it possible to find a setting where oscillations were substantially damped and controllability thus improved. – supercat Sep 30 '15 at 18:50

Hydraulics are used if you need to move something large quickly. Also, hydraulic actuators are perfect for linear motion, whereas electric motors are best for rotary motion. On the other hand, electric motors can be controlled more precisely: By counting the number of revolutions and the angle of the shaft, an electric motor can drive the trim spindle much more precisely than hydraulics, which move fluids into cylinders at high pressure. This does not (at least easily) allow for feedback on the position of the actuator, and only a limit switch can give easy position feedback.

In order to move a number of control surfaces, with hydraulics you need only one pump. Normally, only one surface is moved at a time, so it can use the full power of the pump. Together with a pressure reservoir, this allows for large forces and large strokes. If you wanted to get the same control power with electric actuators, each control surface would need to have a big electric motor and gearbox next to it.

However, hydraulics are messy (can leak) and more maintenance-intensive than electric actuators. Hydraulics are less energy efficient (after all, all those lines create a lot of internal friction drag) and at low temperatures, the oil must be circulated continuously to keep it warm and "runny". That is why electric actuators are now used in applications which had traditionally used hydraulics.

• Drag is everywhere: internal friction drag :-) – menjaraz May 28 '14 at 4:10