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Ramp Ramblings Extra: Trident Autoland

“To land …or not to land, that is the question”

That was indeed the question asked by many pilots up to the late 1940’s whenever the weather was misty or foggy. If they couldn’t see the runway they had to decide when to divert to somewhere clearer for a safe landing. The introduction of ILS (Instrument Landing System) changed all that. This would guide the pilot down radio beams to the runway. However, it was still up to him to decide to land or abort. This decision height was normally made at around 200 feet above the runway, if he could see the airfield he could continue with the landing if not he had to divert elsewhere. As advances were made to the ILS system and aircraft autopilots it became possible to couple the autopilot to the ILS. Such that the aeroplane flew automatically down the ILS to the decision height of 200 feet, where again the pilot had to be able to see the runway to be able to complete the landing manually.

The game changer came in the 1960’s when Smiths and De Havilland developed the world’s first Autoland system for the new Trident being built for BEA. Now not only did the autopilot fly the aircraft down the ILS but it continued to a height of about 65 feet where it would automatically close the throttles and pull the nose up in a pre- programmed manoeuvre into the flare for landing. It would then remove any offset due to cross winds between the aircraft heading and the runway using a system called “Kick off drift”. After touchdown the autoland system would maintain the Trident on the centre line of the runway, using the rudder, until a speed of around 60kts . At this speed there was insufficient airflow over the rudder for effective steering so the pilot disconnected the autoland and used his nosewheel steering to control the aeroplane.

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So how did this all work, well essentially there are two parts, the ground based ILS and the aeroplane based Autoland.

An ILS system consists of three main parts, the Localiser for left /right guidance. The Glidepath for up/down guidance and the Marker system for distance from the runway indication. Let’s look at that first. Three radio beacons each transmitting a very narrow beam are located at set distances along the extended runway centre line. As the pilot flies over each beacon in turn on his approach to land, he will see a different colour light illuminate in the cockpit along with an audible tone in his headset for each marker beacon he passes. This gives him an indication as to how far he has to go before landing.

To avoid interference, the Localiser and Glidepath systems operate in different frequency bands, Loc./ VHF and GP./ UHF . Apart from that their principle of operation is the same. The Localiser aerial is that large structure you can see at the end of the runways at airports whilst the Glidepath aerial is much smaller and is situated at the other end of the runway alongside the landing touchdown point.

Localiser Aerial.

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The localiser transmitter sends out a lobe shaped radio beam which is made up of two smaller lobes, one mainly on the left of the runway centre line the other mainly on the right. If the aircraft is to the right of the centre line the signal strength received from that lobe will be higher than the signal strength from the left hand lobe and vice versa. However, if the aeroplane is on the centre the signal strengths will be equal. This difference in signal strength can be used to drive a pointer in an instrument showing the pilot which way to fly to get back to the runway centreline.

The ILS indicator here is showing the pilot he needs to fly up and left to regain the centre of the Localiser and the Glidepath.

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If the localiser system is turned through 90deg and the centre line tilted 3 deg up, we have the Glidepath system. With both the localiser and glidepath system now feeding their deviation signals to an indicator, this will show the pilot which way to fly left/right or up/down to maintain the perfect on centre line on glidepath approach. Also the signals that drive the pointer can be fed into the autopilot system so the aeroplane can be made to automatically follow the ILS towards the runway.

So that’s the basis of an ILS autopilot coupled approach, what was different with the Trident Autoland? Well because the Smiths system was the first of its kind great lengths were gone to so that safety and reliability could be ensured. Without this the authorities would never have approved its use on an airliner. With its three engines the Trident allowed the autoland designers to come up with a Triplex system such that there were three completely independent channels all working together. It had three pitch channels and three roll channels supplied by three separate electrical and three separate hydraulic systems. Each set of three channels worked on a voting system such that if one commanded the aeroplane to do something different from the other two that channel would be disconnected from the autoland automatically. The “brains “of the system were the three pitch computers which as the aircraft descended were commanded by the Radio altimeters to switch over the various modes of operation as the aircraft approached the runway. These switches were Ledex stepper motors of the same kind used in the old fashioned telephone exchanges!! All three had to switch within milliseconds of each other to avoid a channel dropout. It is interesting to remember the entire system was analogue. The Radio altimeters were very accurate altimeters working on the radar principle. It was this continually monitored reduction in height that was used by the autoland as a measure of how far the Trident was to touchdown.

Autopilot flight controller, the large round switch in the middle top of the panel was the one used to select Autoland.

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At approximately 2000 feet the Trident would capture the Localiser and start aligning with the runway centre line. At 1000 feet if all looked good the pilot engages the land system and the Glidepath would also be captured. As has already been described the Trident would then descend, flare and touch down on the runway centre line. To prevent the aeroplane from floating along the runway the autoland had a “fiddle factor” incorporated into the Radio altimeters in that they always thought the runway was 12 feet lower than it actually was! This resulted in a firm touchdown but ensured the plane would not balloon back into the sky.

As we have seen the entire system was analogue using 28 different computers/sensors for the Triplex autoland. A modern Airbus A320 only has a dual autoland and is all digital. This requires just 6 computers! However, that old steam powered Smiths system was the first and the Trident made the world’s first ever autoland by an airliner in commercial service as far back as 1965.

Some of the Autoland equipment on the Trident

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The Boeings that replaced the Trident in British Airways service in the 1980’s had autoland but their systems were not approved down to such low weather minima as the Trident’s CAT 3B Autoland (Pilot not required to be able to see the ground i.e. no Decision Height and a forward view of just 150 feet). In fact, when the brand new Boeing 737’s arrived from Seattle BA management arranged a VIP flight up to Scotland for lunch. However, when the well replete guests returned to their Boeing for the return journey they were dismayed to be told that fog had closed in at Heathrow and they would be delayed whilst a Trident came to their rescue as the Boeing’s state of the art digital autoland could not cope with such bad weather!

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So there we have it, no longer did BEA crews have to ask “To land or not to Land”. It was always a case of “prime and engage land” and complete another successful autoland whilst other aeroplanes were grounded. Smiths and De Havilland had made the Trident and Great Britain a world leader.

If you have found this article interesting and would like to see the Trident autoland in operation, go to YouTube and search “BEA Hawker Siddeley Trident 1c promo 1968” for a 20min video.