Gnat Pitch Control System
The Gnat does indeed have elevators, but in normal flight these are 'locked' to the tailplane. Therefore, any pitch input will move what is effectively an all-moving tailplane.
Pitch inputs from the control column are fed to the hydraulically powered tailplane actuator via a Cam mechanism and Q-gearing mechanism. The Cam mechanism simply alters the rate-of-change of output depending upon the input position; the further back the stick the greater the tailplane movement per degree of stick movement. The Q-gearing mechanism modifies the elevator gearing dependent upon Q, effectively IAS. An electrical-operated feel trimming system is also fitted.
A datum shift mechanism is also provided. Due to the large nose-down change of pitch caused by the undercarriage lowering, the system simply shifts the datum position of the pitch system as the undercarriage is lowered, i.e. applies a small nose-up input. Simple but effective.
In normal use the resulting pitch input is feed into the hydraulically powered tailplane actuator moving the tailplane and locked elevators as a single unit.
The elevators can be unlocked from the tailplane in flight by a lever in either cockpit. The elevators are now 'manually' controlled directly from the stick, i.e. with no hydraulic assistance, but the hydraulically powered tailplane continues to 'follow up' any pitch inputs. Following loss of hydraulic pressure, or manual selection of 'TAIL & AILN OFF', an accumulator will provide a very limited reserve of pressure before the tailplane system freezes leaving a limited amount of pitch control available directly through the now unlocked elevators. Limited trimming is now only available by a standby electric motor which trims the aircraft by moving the tailplane.
Depending upon configuration, i.e. IAS, gear up or down and the position of the tailplane, it is quite possible to run out of the rather limited elevator authority.
Following a loss of hydraulic pressure in normal flight it is imperative that the elevators are unlocked promptly as part of the STUPRECC drill.
I seem to recall a display board at RAF Valley in the ground school tea bar with the Gnat elevator system layed out in detail.