TrevBrum

GUIDED LANDING SYSTEMS

Introduction.

In the years before the second world war the success of a deck landing depended almost entirely upon the skill of individual pilots. There was hardly any definable doctrine and very little help from the carrier to guide an incoming pilot. They were well trained and supposed to know. To be fair, they usually did, given the calibre of the fliers, the low speeds and generally docile flying characteristics of the aeroplanes of those days. Something like the standard airfield approach, but with power on to catch up with the carrier, though unable to “fishtail” to adjust height, enabled a well judged landing to be quite gentle ([Ackland] in references). With no arresting equipment or wheel brakes, it was then for the deck handling parties to bring the machine to rest and keep it under directional control as it ran along the deck.

Adverse weather and severe ship movement posed their problems but these were perhaps avoided rather than faced in those comparatively peaceful days. After all, flying between the wars was supposed to be fun.

But this soon changed and, from the mid 1930s, machines began to appear with greatly increased performance, more complex, with more powerful engines, flaps, brakes, more demanding handling characteristics and enclosed cockpits separating the airman from his environment. All these advances called for new techniques to match. However, as they appeared in naval aircraft somewhat later than in those for the RAF, experience was readily available, for many naval pilots were at the time still being provided from that service.

The 1930s, with this increasing rearmament, saw the recruitment of large numbers of new aircrew and, in 1938 came promise of the long desired assumption of full control by the Fleet Air Arm over all of it’s own activities, freeing it from RAF tutelage.

New methods of training suited to the high throughputs of pupils were introduced, although initial pilot training was still being carried out alongside the R.A.F. recruits in their Elementary Flying Training Schools right up to the outbreak of war in 1939.

The introduction aboard ship of Deck Landing Control Officers, better known as Batsmen, was another major step and a much needed innovation at the time. All these advances marked the final demise of the individualists of the earlier period.

As the war proceeded, techniques were constantly refined and by it’s end, in 1945, had become well adjusted to pilots of limited experience, with tail wheeled, piston-engined machines, landing under the guidance of these batsmen who were, of course, experienced pilots themselves. A steepish descending approach at a speed a little above the stall, followed by an engine cut and flare out would, when executed well, produce a good landing, comfortably within the available area, at minimum possible speed and hence with minimum stress imposed upon the aircraft and ship’s arresting gear.

But increasing entry speeds were calling for ever higher standards of skill and faster and faster responses, by batsmen as well as by pilots. Even in the less pressured times after the end of WW2, the accident rate went on rising as ever more advanced types of machine were coming into service.

The Sea Fury in the early 1950s was an example, incorporating the rapid improvements in heavy fighter bombers forced by the war. The number of landing accidents with it became a serious concern. It did not have any serious handling vices, but an approach speed around 100kts, with its weight and pilot visibility limitations, were enough to highlight the difficulties.

Neither was the technique itself the only difference between the two R.N. and USN. Their batsmen’s signals had an unfortunate inbuilt conflict as well. In the RN these were mandatory. Arms up meant “Go Up”. In the American case, this same signal meant “Too High”; that is, an instruction to come lower. The potential for chaos was there and duly asserted itself when cross operation became more common, as it did towards the end of the war and into the cold war. The decision was made to adopt the U.S. approach methods in the R.N. in 1948 and, after the expected teething troubles, accident rates began to fall.

Such were the procedures existing into the early 1950s. With the advent of jet aircraft it soon became evident that these modest improvements would only be a respite.

Mirror Sight Concept.

The first generation of service jet aircraft were appearing in naval form, heavier and with still higher landing speeds. Tricycle undercarriages and enormously better visibility for pilots might have mitigated the difficulties somewhat, but the limits of batsmen’s functions began to show.

Looked at as a Servo Loop - i.e. pilot’s action; followed by bat’s appreciation; followed by his signal; followed by the pilot’s translation of that signal; then his correction; was becoming just too cumbersome with planes approaching at over 100kts.

At the same time, the standard approach was also much more difficult to judge correctly in the new conditions. This was partly because of the slow response of the early types of gas turbine to a sudden cut in power, made worse by the aerodynamic refinement of the jets themselves, which had no windmilling propeller to help slow them when the throttle cut, and often tending towards marginal control responses at this vital time. What had come to be needed for these machines was a firm push over followed by a flare, and this called for considerable skill. The chances of arriving too hard or, on the other hand, not touching at all, were pretty well evenly matched. Re-powering of those early jet engines was also very tardy once revs had been lost, making any correction hazardous, and so the tendency was for pilots to hold up the revs, and hence residual thrust, right down on to the deck.

The then Commander(R.N.) H.C.N.Goodhart, both a Boscombe Downtest pilot and an engineer, began to consider the problem. He thought that a steady, constant throttle, straight descending approach right down on to the deck might be feasible if it were possible to select a descent path which would eliminate the need for a flare out but still remain within limits of the undercarriage. He also saw it as essential to break into the servo loop by eliminating the batsman and presenting approach information directly to the pilot.

Figs 45 a, b and c show the result of his thinking on the first point.

Fig 45 - Approach angle as affected by ship pitch

a) Shows the carrier in calm conditions and an approach path meeting the above requirements.

b) Shows the effect of the ship pitching, Bows DownStern Down. The aircraft now strikes the deck at the acceptable descent angle plus the ship pitch angle.

c) Shows the ship pitched Stern Down. Round down clearance now becomes the critical factor, the angle of approach relative to the deck having been reduced

Given some allowance for deviations from these ideals by both man and equipment, Cdr. Goodhart concluded that a no-flare approach in any reasonable sea state was achievable. A basic angle of approach of 3^o seemed to meet the requirements in either of the two ship pitch modes, and was finally decided upon. It then remained to devise a system to ensure that a pilot could follow such a path in space, irrespective of ship motion, with the necessary accuracy. Gyro stabilization of any apparatus would be essential to ensure this.

The thinking was greatly helped by the concept of the angled deck, which was under consideration at about the same time. Indeed, it was in discussion with the co-inventor of the Angled Deck, who happened to be his immediate senior, Captain(RN) Denis Cambell, that this optical solution emerged.

It was first demonstrated by borrowing a handbag mirror from the Captain’s secretary. Suitably angled and with a horizontal datum line drawn across it, it reflected the tip of her lipstick tube, placed a little in front of it. By keeping the reflected image right on this datum line, an approach path could be followed by eye right down to the desk top as shown in the drawing Fig 46 below.

Fig 46 - Mirror Landing Aid. Concept

The idea was rapidly adopted and then developed into a practical deck landing aid - the Mirror Landing Sight - at R.A.E. Farnborough. A sketch showing the principle as it was applied aboard ship is at Fig 47.

Fig 47 - Mirror Landing Sight. As applied on board.

The large mirror was mounted at the side of the flight deck and aimed aft, along the flight path.. The approaching pilot saw the reflected image of a powerful line of lights which were positioned at the deck edge 150 feet aft of the mirror and aimed forward towards it. The mirror was tilted from the vertical by an amount necessary to project the reflection of the line of lights at the desired approach angle, about 3^o, and was also gyro stabilized to hold this against ship pitching motion. Two arms, carrying lights, were added on either side of the mirror to effectively extend its centre line. They formed the horizontal datum either side of the mirror.

If the pilot saw the reflected lights in line with the datum, he would be on the correct path.

If it appeared to be above, he was too high and if below, too low.

Provided that he kept the light lines in proper relation, he would be guided right down on to the deck, could concentrate on his approach and virtually ignore the ship, all the time flying just above stalling speed. Engine power only needed to be cut when he sensed wire engagement. A further refinement, fitted later on the aircraft, was an airspeed indication reflected on to the windscreen, head up, to save the pilot looking away from the mirror sight whilst flying down the beam. This was later improved yet further to give an audible signal in his earpiece.

The mirror was made slightly concave about it’s vertical axis, so as to widen the line of sight in azimuth. The image could then be picked up as soon as the pilot turned in to the crosswind leg.

Trials on H.M.S. Indomitable, carried out in 1953, showed the potential of the system and more experience, at Boscombe Down and in H.M.S. Illustrious, further confirmed these promising results.

The benefits were listed as being:

1) Effectively halving the element of human error. The related development of head up, then audio, airspeed display played its part.

2) No change of attitude and no need for a sudden engine cut upon crossing the round down. Important because of the inability of gas turbine engines in those days to regain power quickly from idle.

3) Unchanging perspective of the deck and absence of “cliff edge” effect over the stern. Distracting ship movement almost totally masked. The carrier itself could virtually be ignored on the approach.

4) Rates of descent at touch down reduced to an average 12ft/sec., with potential to reduce to 8ft/sec. A relief for undercarriage designers.

5) With improved dimming arrangements, all these benefits applied to night landings. The approach legs of the circuit, now at a more or less constant 500 feet instead of 150, could be flown with a much higher factor of safety in the dark

6) Far greater visibility of these lights than of D.L.C.O. bats, even more so in foul weather.

Fig 48 - Mirror Landing Sight deck unit.

Such was the enthusiasm that fully engineered ship sets were put in hand and fitted as quickly as possible. The concave mirror, now reduced to four feet square from the prototype eight feet, made from polished aluminium and mounted in a cast aluminium frame, was set well forward on the port side of the flight deck. Datum lights either side were coloured green and the aiming light source, 150 feet further aft, had evolved into a row of amber coloured lights. Two red emergency wave-off lights, operated from Flying Control, were placed apart underneath the mirror.

The mirror frame could be raised or lowered through 4 feet, to allow for variations, in different aircraft types, between the pilots eye level and the hook level. This because the pilot followed the beam with his eye, the hook which engaged the arresting wires could be up to twenty feet below in the vertical plane, as well as many feet aft. The aim was to pick up number five wire, plus or minus two, out of the twelve usually deployed, the mirror dimensions having being decided by this consideration. The unit could be set at any desired approach angle and this could be done at the unit itself by means of handwheels, or remotely by use of Magslip followers.

A Service Type ”P” gyro, locked to the horizontal datum, was incorporated, with a mechanical back up in the event of failure. The developed unit is shown in Fig.48.

Together with the Angled Deck, the Mirror Sight contributed to a quantum jump in the safety of naval flying. The accident rate fell to less than fifty in ten thousand landings. A further huge benefit gained was that, because of the more precise landings now achievable, fewer arresting wires with their associated heavy under deck gears, had to be fitted.

Residual Problems with Landing Aids

However, all was not to be perfect in practice, in spite of the great advantages gained from the Mirror Sight.

One problem was that, with the Angled Deck, the Mirror, well offset from the deck centre line, being un-stabilised in roll and aligned to the fore-aft axis of the ship, caused an error to be introduced when the ship rolled. A 3^o roll on the carrier could cause an approach path error of ½^o.

With a 9^o angled deck, the aircraft would be offset 450feet to starboard from the ship’s wake at 1000 yards out. The aiming line could be discerned by the pilot up to 40ft above or below his correct flight path in this position. A 3^o ship roll might cause a 60ft diversion of the beam centre and it would appear to move up and down even on a steady correct path and, unless the approach was near perfect, might disappear altogether for short periods.

So ship pitch and roll continued to complicate the landing. A great deal of thought was given to the possibility of recording these motions and, because they were to some degree regular and cyclic, for Flying Control to try to predict the ideal moment to touch down. The aircraft position on the approach could be assessed as well as its approach speed. It was thought that, from a continuous read out of ship motion, it might be possible to foresee the next cycle in which the landing deck would be near level at the time of landing. Otherwise, a wave off would be signalled. It might even have been possible to automate this, using a Doppler radar, thereby relieving the Commander Flying of the responsibility for making such snap judgements. It is not known if this was ever achieved.

Furthermore, there were also difficulties for pilots in keeping sight of the deck centre line in times of poor visibility. There was usually a broad yellow painted band which required constant cleaning and renewal, and could often not be seen at all in night operations The problem was not made easier by the somewhat crablike approach necessary to the angled deck, although if the meatball was kept in view at all times and followed instinctively this would not be too serious.

Various attempts were made over the years to install more effective centre line lighting. Lights inside small covers were regularly picked up by the aircraft hooks and became a maintenance nightmare. Two bright sodium lamps, one on the round down and another on the forward edge of the angled deck were then tried. The latter had to be made retractable for safety reasons and, as it sometimes didn’t, frangible mirrors were tried with little better success and the idea was finally abandoned. Sodiums were then installed along the port side of the flight deck and this half dealt with the problem, although it could cause interference with the pilot’s view of the mirror itself.

What was needed was a clear lit line installed flush on the deck with distinctive colour and variable intensity, but no perfect solution had been found by the time the U.K. finally withdrew from carrier building.

It would have solved some of these problems if the Mirror Sight source could have been projected from the centre line of the deck so as to avoid these parallax and cross-coupling effects. As far as is known, nothing was ever devised to do this.

Projector Sight.

By the mid 1950s, there was the prospect of a more effective second generation light guidance system. Proposed by G.E.C., a British electrical engineering company, it was designed to dispense with the remote source lights and to replace the mirror with a vertical array of lamps aimed directly along the flight path.

Known as the Projector Sight, it used Fresnel Lenses which, by special profiling of their glass, gave narrow and high intensity directional beams. It became therefore a self-contained unit, albeit of much the same dimensions as the mirror structure.

These next two figures show the essentials of the unit that was eventually developed.

Fig. 49 - Hi Lo Projector Lamp.

Fig 49 - Hi Lo Projector Lamp.

Fig 50 - Lamp array in its unit.

Fig. 49 is a sectioned diagram of one of the lamps

The presentation was directed so as to guide the landing in two phases, namely the Outer and the Inner Approach. A diagram of the series of Outer Approach signals seen by the pilot, and depending upon his flight path, is at Fig 52.

When the aircraft was still well out on the circuit, the Outer guide lights came into sight, projected from enclosed boxes and called the Hi Lo. A multi-coloured array would be seen, changing progressively from red to white through pink, depending upon the pilot’s positioning above or below the correct flight path. Too high saw white; on line, pink; and too low red. This display was visible up to 45^o either side of the flight path, the wide band being necessary to assist homing from out in the circuit, somewhat before the last turn in.

These coloured Outer Approach signals were generated by mixing bars of red and white light from the “Hilo” indicators. The Hilo box which produced them was in effect twinned, a row of six lamps above for white and another six below for red.

Fig 51 - Lamp array as seen by pilot.

Near to the correct flight path, light from both parts would be seen, mixing into progressive shades of pink, with a gradual transition from white to red from ½^o above to ½^o below that path. Provision was made for dimmers to prevent dazzle in certain conditions, by aiming-off the lamps.

As he came into less than about 3,000 yards, the pilot would begin to pick up the Inner Approach guide and to see a yellow spot of light which moved up and down in relation to a ¾horizontal bar of light either side of it, the datum bar. This was the indication of departure from the ideal path, similar in function to that in the Mirror aid, giving guidance to within plus or minus 3/4^o in azimuth.

To produce these graduated Inner Approach signals, the main projector box was fitted with twelve Freznel Lens lamps set in a line vertically one above another, shown in Fig. 51. Each lamp threw a beam which was wide horizontally but very narrow vertically. They all had the same width, but in the vertical plane each light overlapped the next by a small amount, so that beams from three adjacent ones were all that could be seen by a pilot from any one position within the guidance zone. When on the correct path, only the centre beams were visible, giving a bright spot. Moving off this, the beams above or below began to come into view and the spot appeared to move and because of a slight defocusing it appeared to move quite smoothly as the pilot’s course diverged.

Each lamp had an elliptical reflector, a colour filter and a lens in front of it. The upper ten showed yellow colour and the bottom two, red. The horizontal spread was 25^o to starboard (port relative to the ship’s head) and 15^o to port, total 40^o.

Individual slits in front of the lamp arrays were all built on to a single, very light, slide carrier, which could be set at the required angle and would readily move up or down to correct for ship pitch, to a limit of plus or minus 5^o.

The whole unit could be raised or lowered over 6 feet to allow for various “hook-eye” distances. Stabilization was as for the projector slide. Two red wave-off lights were fixed atop the datum arms. The complete array, incorporating both Inner and Outer guidance systems, were mounted alongside each other and positioned two hundred feet forward of the deck aiming point to minimise the parallax effect of the offset to the side. Usually to port, it was adaptable to the other side of the ship as well.

Worked up at [R.A.E. Bedford], the equipment had first sea trials in HMS Bulwark in the early 1960s, thereafter being progressively refined, superseding the Mirror Aids and remaining in service with the Royal Navy until U.K. Carrier operations came to an end and in the U.S. Navy, perhaps in more refined forms, ever since.