Movable horizontal stabilizers
As noted in Chapter 4 helicopters have a horizontal tailplane to improve both manoeuvre stability and dynamic longitudinal stability. Sometimes these surfaces need to be large in order to provide the necessary influence. However, a large tailplane can itself engender several problems such as excessive TCWP, excessive download (and associated poor performance), pitch-up during low-speed flight and high nose-up pitch attitudes in the hover. At the expense of weight, complexity and reduced reliability, a number of these problems can be overcome by making the tailplane incidence adjustable under the command of the AFCS. Such devices are called programmable stabilizers or, in American parlance, ‘stabilators’. Movable horizontal stabilizers will be discussed separately from the host AFCS as only a few helicopters are fitted with these devices and they offer a range of novel AFCS enhancements and some unique failure characteristics.
There are several reasons for incorporating stabilators: some associated with enhancing performance whilst others overcome the handling problems often associated with large tailplane surfaces. The key reasons are listed below: 
• To improve ‘cockpit’ longitudinal static stability (both collective fixed and apparent) by programming the stabilator trailing edge up (TEUP) as airspeed increases thus causing the pilot to have to apply more forward cyclic to stabilize at an increased IAS and vice-versa.
• To improve longitudinal dynamic and manoeuvre stability by programming in response to a pitch rate signal and, possibly, a pitch attitude signal.
• To reduce TCWP by programming trailing edge down (TEDN) with increased collective and vice-versa. As this function is ineffective in low speed flight it is often phased in with airspeed.
• To oppose the effects of a pitching moment due to sideslip (Mv) on aircraft with a canted tail rotor by programming in response to a sideslip or sideforce signal.
• To improve FOV by programming, or being commanded, TEDN at NOE airspeeds, typically 30 to 70 kts, thereby reducing nose-up attitude.
Increasing the stick migration with speed can be matched to a reduction in the pitch attitude change with airspeed that may result in improved crew and passenger comfort due to a level fuselage deck in high-speed cruise flight. Equally performance may be improved since the relatively level fuselage will typically produce less drag although the extra trim drag from the stabilator will tend to reduce this effect. However, reducing the variation of pitch attitude with airspeed is not always beneficial. The pitch attitude hold will be less effective at maintaining airspeed leading to a requirement for an actual airspeed hold function which, unfortunately, is not always satisfied.
As stabilators are normally large and in the hover they may deflect up to 45° trailing edge down any failure that could cause it to runaway TEDN at high forward speed could be potentially catastrophic. The large nose-down pitching moment generated would probably exceed the available cyclic control power even if the pilot were able to react in time. A similar, although potentially less serious, trailing edge up failure case also exists. Consequently, even though stringent reliability requirements are applied, the stabilator slew rate is usually a compromise between normal operation when a fast rate may be necessary and the failure case when a slow runaway is desirable. Most stabilator systems are thus duplex and feature extensive safety and monitoring devices. These normally take the form of a comparator that checks the positions of the duplex actuators and the signals that drive them. A mismatch typically provokes a complete stabilator freeze and an aural warning. Once the automatic functioning has frozen the stabilator can usually be controlled manually provided there has been no mechanical seizure.
Although normally controlled by the AFCS, most stabilators have a manual mode which may be used in flight to give a measure of control over pitch attitude and, possibly, vibration. As controls for this type of operation need to be accessible from the primary inceptor they are normally placed on the collective where they may be operated by the left thumb. A safety device may be fitted to prevent manual operation above certain airspeeds to prevent the inadvertent overpowering of the available cyclic control. Some installations feature a semi-automatic NOE or approach mode which, when preselected by the pilot, will cause the stabilator to slew TEDN according to a revised schedule designed to cause a nose-down pitch attitude and so improve the FOV.
In an emergency, following a system freeze for example, it may be necessary to slew the stabilator manually. In this case, the stabilator is set to a fixed position that corresponds to a particular airspeed schedule as indicated in the flight reference cards or pilot’s notes. This is normally a simple selection of stabilator level above a certain airspeed or maximum TEDN below about 40 KIAS. With a frozen stabilator dynamic functions such as rate damping and sideslip correction are no longer available and so a minor degradation in aircraft handling qualities usually results. Prompt pilot action may be required to prevent a disastrous nose-down tuck if the stabilator should fail to programme correctly during a rapid transition from the hover. As the pilot will have little time to identify and operate the slew-up switch one is usually incorporated into the cyclic grip. The switch is usually designed so that pulling it aft causes an immediate slew up. This is the natural sense in terms of its action on the pitch attitude of the aircraft.
The stabilator is normally programmed by an airspeed signal derived from the normal pitot-static system; its movement will thus be subject to any PEs that may be present. This is particularly important at low IAS where pressure errors may adversely affect stabilator programming during transitions, which is exactly when the fastest and most accurate programming is required.