The Director Horizon was an artificial horizon with two additional bars. Their movement echoed calculations by the S.E.P. 2 autopilot, governed by whatever settings had been made on the control box. The pilot could either use them as a guide, or engage the control servos and let the aircraft follow them automatically.
The Beam Compass showed a plan view, like a modern horizontal situation indicator. On the face, a large needle represented the current heading, a smaller one the pre-set heading for the autopilot, and a bar represented a V.O.R. or localiser beam. Around the bezel, a sense knob determined whether the instrument should pick up forward beams, aft beams, or both, another knob rotated the compass card, and there was a warning light if something went wrong.
Integration allowed new checks and balances: two gyro-units supplied the pilot and the co-pilot's panels, along with their own sets of static and dynamic sensors. Normally, both sides would read the same. However, if instruments on either side disagreed, warning flags and lights would alert the crew. With analogue systems, that was all they could do. The crew still had to decide what to do about it. It wasn’t until the advent of the digital computer that a flight system could follow logical steps, let alone any kind of decision-making process.
In its day, the S.F.S. was revolutionary. B.O.A.C. and B.E.A., who had been closely consulted during development, immediately specified it for all their new airliners: the Bristol Britannia, de Havilland Comet, and the Vickers Viscount and Vanguard. A military version, known as the Military Flight System, was fitted to Vulcan and Victor bombers.
It wasn't without criticism: on the director horizon, Smiths opted for two separate needles, one for roll and one for pitch, as opposed to the more conventional, all-in-one, ball. On the beam compass, the needle moved as the aircraft changed course, not the compass card. Smiths claimed it was better in zero visibility, and had done tests to prove it. It was also true that pilots who used it all the time grew to appreciate it, but pilots who flew
| De Havilland DH-121 Trident, fitted with a Smiths Flight System and Autoland, on approach at Hatfield. | FLIGHT |
lots of different types of aircraft found it counter-intuitive, and hated it. Considering the whole point was to reduce mental gymnastics, it was an eccentric choice, and damaged sales against competing systems from Sperry, Collins and Bendix.
However, the system was very successful, technically. Following years of parallel research at the Blind Landing Experimental Unit in Bedford, a Hawker-Siddeley* HS-121 Trident, equipped with an advanced Smiths Flight System, made the world’s first fully-automatic landing during a commercial flight, on 10 June 1965.
*The Hawker-Siddeley Trident was designed for British European Airlines by de Havilland as the DH-121, before the merger with Hawker in 1960. The historic landing was at Heathrow, using G-ARPR, commanded by Captain Eric Poole.