What classes of aircraft require hydraulic flight controls?

Question

All commercial airliners such as the Boeing 737 or Airbus A320 or heavier aircraft require an hydraulic system that can power actuators, which generate the required forces to move the control surfaces in all phases of flight. Smaller General Aviation aircraft do not need this.

But what about aircraft in between? Is there a minimum MTOW above which aircraft are required to or generally have an hydraulic system? Can you make examples of lighter aircraft which have it?

For example I was wondering about the an aircraft like the 19-passenger commuter Beechcraft 1900:

Answer 1

Hydraulic flight controls are not mandatory for a specific class, but above some size and speed it will be hard to fulfil all certification requirements without at least adding some hydraulic boost.

Historically, aircraft used manual controls. With increasing size and speed, the forces went up and increasingly clever force reduction systems were invented to keep stick forces within limits.

• On the first flight of the Messerschmitt Me-321 glider it was found that control forces especially in roll were way too high, so a second pilot with a second stick was installed.
• The Dornier Do-335 or the Republic P-47 used a telescopic stick: At low speed the stick was short and could move through the full range of aileron deflections while at high speed it could be telescopically extended. This limited the motion range but improved the leverage of the pilot.
• The English Electric Canberra used internal balances for the ailerons and a spring in the aileron controls to limit the stick forces. At top speed the control horn could still move through its full range but the achievable aileron deflections were only a few degrees - still enough for sufficient roll agility, however.

Ailerons have mass. There is a spring. Something that moves. Every engineer should think next: What about flutter? With more complex force reductions, the chance grew that something unexpected happens and failure modes multiplied. In the end, introducing hydraulics solved so many problems that de facto above a certain speed and size combination no other option makes sense.

I have worked together with Rockwell (formerly North American) on a two seat jet trainer; a size and speed combination that would have had manual controls 60 years ago. I proposed to use the same force reduction mechanisms that were used on the F-86, but in the end a hydraulic boost was selected. Why?

• Nobody had any experience with manual controls anymore. After so many years, all F-86-era engineers were long retired or had passed away.
• The North American archives had been thrown away years before, so no documentation was left on how to properly design a manual control system for that speed range.
• The hydraulic boost system was easy to install, used standard components and was cheaper to design because everybody was familiar with the technology.

As I said before: Don't get me started on how exactly you can tailor roll stick forces - this will be a post that will be much longer than anything I have written here before.

Answer 2

As far as I know there is no hard regulation requiring hydraulics at a certain point. On one hand the aircraft need to actually be controllable so if that would be unattainable at operational speeds then hydraulics would bee needed.

In the FAA's Brief on flight controls

As aviation matured and aircraft designers learned more about aerodynamics, the industry produced larger and faster aircraft. Therefore, the aerodynamic forces acting upon the control surfaces increased exponentially. To make the control force required by pilots manageable, aircraft engineers designed more complex systems. At first, hydromechanical designs, consisting of a mechanical circuit and a hydraulic circuit, were used to reduce the complexity, weight, and limitations of mechanical flight controls systems.

The FAR's list the limits of force on a control input in FAA FAR 23.397

(a) In the control surface flight loading condition, the airloads on movable surfaces and the corresponding deflections need not exceed those that would result in flight from the application of any pilot force within the ranges specified in paragraph (b) of this section. In applying this criterion, the effects of control system boost and servo-mechanisms, and the effects of tabs must be considered. The automatic pilot effort must be used for design if it alone can produce higher control surface loads than the human pilot.

Keep in mind this does not require hydraulics outright but if you can not build a cable/trim tab system that falls within these force limits you will need to add in some kind of assistance.

Answer 3

There isn't a particular weight or size class for requiring hydraulic flight controls. The requirements are for control forces to stay within certain bounds, and in general this becomes more difficult with increasing size and speed of the aircraft.

Keeping the control system manual has the advantage that the aeroforces are fed back to the pilot, and that with increasing airspeed the controls stiffen up, making it harder to making large deflections which would impose high accelerations on the airframe. But larger/faster aircraft require larger control surfaces, with the associated larger control forces. However the control forces must remain within the boundaries of human force ability.

Many of the older hydraulically actuated aircraft have some form of feedback of aircraft velocity. The 737 has q-feel in the pitch channel: an artificial feel system stiffens up the flight control as a function of dynamic pressure as sensed by the pitot tube. The DC-10 and the L-1011 have this as well.

In a previous life I used to contract for simulator modelling of flight control systems of fixed wing and helicopter flight controls, and the Beech 1900 is one of the types that I worked on. It has a fully manual control system, all aeroforces are fed back to the pilot. The pitch channel has a down spring of about 40 lbf, which is apparent from ground measurements. Have heard comments about the pitch forces being high during landing, and that particularly female pilots can have trouble exerting these forces during landings. Hydraulic actuation may make it easier to fly the aircraft and tailor the required forces. There are larger and faster aircraft certified for manual control, the F100 is one: pitch has manual backup.