Radio Navigation Systems

Lorenz/ Knickebein

RF/DF Direction Finding

 Beam Approach

 Q Code and Identifiers

 

Pip Squeak

 

       
This page gives a history of the ZZ landings system. The above links continue with other Radio Aids

 

The ZZ Landing System

 

Most Bomber Airfields were situated in low lying land in Eastern England and frequently subject to adverse meteorological conditions.

In the worst circumstances even the nominated diversion airfields could be fogged in and Blind Landing methods were explored.

The USA and Germany had explored various possibilities but the UK with its legendary bad weather had tried tethering a balloon to identify the airfield hiding below in the low cloud or experimenting with trailing a weight beneath the aircraft  which would trigger a light in the cockpit when it struck the ground.

Eventually the RAF adopted the ZZ landing system and  RAF ORB's (Operations Record Books) 1940-42 or Pilots Log Books variously mention on Blenheims and other aircraft of the time  '... ZZ practice was carried out' .

 

Example Entry

In October 1941, Blenheims practiced numerous tests at Abingdon on ZZ, also with Lorenz. This was also practiced in December 1941, and approach practices on ZZ to Abingdon continued in January - March 1942.

 

 

From my brief research and some knowledge of flying an aeroplane I can understand how difficult this was for the pilot even with a radio operator. I believe it was almost impossible  for an average RAF solo pilot due to the work load but records show some had success  .

 

In Pre War Commercial Aviation,  The Airline Lufthansa seemed to be the instigator off ZZ landings in 1933.

 

The ZZ landing system was essentially done by morse radio transmission and not as often described as being a radar system.


The system relied on a ground controller, based in a hut at the upwind end of the runway initially using RF/DF direction finding to guide the aircraft overhead the field and having transmitted by morse the QFE (barometric pressure at airfield height).

The pilot would ensure a height of not less than 500 feet (this documented fact seems rather too low to me) and confirmed overhead by morse (QFG) by the operator listening out.

The pilot would then perform a 180 degree turn plus 30 degrees and fly away from the active runway threshold using a stopwatch for a period of 8 minutes before turning through 210 degrees and timing in on a very shallow descent to final approach.

All the time during the approach the QDM (course to steer) was advised by the operator on receipt of a radio signal.
The pilot had to be very experienced, as did the ground controller. They had to have total confidence in each other - this was rarely the case. The controller also had to physically 'see' the aircraft just before touchdown. Hence it was no good in very bad visibility normally a 300 foot cloud base was the limit although totally blind landings were carried out probably by accident.

It's called ZZ as, on final approach the ground controller would send a Morse 'ZZ' if it was OK to land, and 'JJ' if an abort was necessary (Why on earth these combinations - dah-dah-dit-dit, or dit-dah-dah-dah.  What's wrong with dashes if its OK, immediately turning to dots if it isn't - I thought common-sense was in use until the late twentieth century).


It was widely used at commercial airports in Europe and at eight aerodromes in the UK pre WWII

ZZ probably became obsolete when VHF-Blind Approach was developed c.1941 - it allowed the pilot of a single-seat fighter to interrogate a ground 'beam' beacon using his existing R/T set.

 

 

Another description of ZZ from AVIA2/1263: AERODROMES: Government, Municipal and Private

Croydon Airport: ZZ system: proposed installation of (1938-39):

First, the aircraft is brought over the aerodrome by D/F bearings.

 

The pilot turns away from the aerodrome on such a course that he has calculated beforehand will make good a track of 8deg. of the Schneise. 

A Schneise is a descriptive term from the German for a swathe or clearing in a forest and as such free of trees or obstructions.

As he does so he starts a stopwatch. He aims to arrive at a point over the ground from where he can turn on to the Schneise for the final approach, the turn taking him right on to the Schneise should be 1.5 minutes and at the point where he reaches the Schneise he aims to have 7 minutes of flying to take him to the aerodrome, his speed from the moment of arriving over the aerodrome till he lands being taken on an average as 90 miles an hour.

He must therefore have calculated beforehand as correctly as possible his outward flying time and course and his inward course to give an inward flying time down the Schneise of 7 minutes, the time being taken with a stopwatch. The aeroplane is kept on the outward "track by QDR bearings received as often as possible by the radio operator.

QDR is the morse request for Reciprocal Magnetic Bearing (opposite of QDM)
If his bearing is greater than it should be, he will lessen his course, and vice versa. At the end of his calculated outward time, he turns on to the Schneise. As soon as he is on his approach course he begins to lose height. The operator asks for QDM bearings (direction to steer) as soon as the turn is finished. Height is lost steadily all the way down to about 300 ft.

QDM bearings are obtained as often as he can get them. If the bearing is greater than the correct Schneise course he increases his bearing; if the bearing is less, he decreases his course. (NB. Correction in this respect is opposite to when he is receiving QDR bearings.

If the correct course has been held at the end of approximately 7 minutes a message "Engines" in the correct direction is given.

The Pilot then knows he is approaching the aerodrome from the right direction and he can then lose height still further.

On receiving the final signal "ZZ" the pilot knows that it is alright for him to shut right off and land straight ahead.

This is the procedure so long as everything goes alright, but as may be imagined, this is by no means always the case and frequently the signal "JJ" has to "be given to signify that the pilot cannot land.

There is no doubt that before a pilot is really qualified to bring a machine in with a really low ceiling by this method he requires plenty of practice.
The direction and the amount of correction require thought and practice, as does the moment when to turn back on to the corrected course.

Lufthansa have evolved certain rule of thumb formulae for the correction of errors when the pilot gets off his track either on the outward or the inward flights, but it is only by actual practice that the pilot will really learn how to keep to his track.

ZZ had been installed at Heston. The general feel on reading this document was that the new-fangled Lorenz system without the need for the ground operator was to be far superior in commercial aviation, and ZZ was by then considered 'old-hat'.

In the RAF it was also replaced by Lorenz  (SBA -Standard Beam Approach ), but certainly c.1940 there was no way SBA  or any other system would fit into a single seat fighter.
 

Q Code used

QFA    What is the meteorological forecast 

QFE    Height established by barometric pressure above airfield

QNH    Height established by barometric pressure above mean sea level

QFG    Am I overhead

QGH    May I descend below the clouds

QDM    Request Direction to Steer

QDR    Reciprocal Course to Steer


 

From Flight Magazine Archives  May 4th 1939 Pages 461 to 464

Part 2 : The Technique of Blind approach and Landing by Frank Brent

 

THERE are three methods in use by which a blind approach can be made ; namely, by a series of bearings and a descent to below the cloud ceiling when near or over the aerodrome, by the " ZZ " method and its variations, and by the aid of an approach beacon.

 

The first method can only be recommended with a ceiling of at least 100 metres above the highest obstruction in the vicinity of the aerodrome, for there is a lack of definiteness about the information which can be given to a pilot as to his position over the aerodrome. The only method practicable for this is for an observer on the ground to send signals saying where the noise of the aircraft's motors can be heard, and such signals are by their nature very approximate and convey no information about distance. If this method has, unfortunately, to be adopted, the following procedure would appear to be more suitable than a random descent somewhere near the aerodrome on " motor signals." It can be used with ceilings much below the height of local obstructions if the approach is made on the best approach line and the pilot is absolutely familiar with the topography of the place.

On receipt of Turn No. 1 a bearing and distance from the aerodrome are obtained so that the time and arrival over the aerodrome will be known in advance. Approach is made on a series ot magnetic reciprocal bearings whilst descending to a height which will give a clearance of at least 150 metres over any local obstruction or high ground. The direction of the preliminary approach is best arranged so that it will lead across the aerodrome towards the lowest and most obstruction-free area, but if this cannot be done without making a special detour it is not really worth while, as it must be remembered that time is an important factor, and there are probably other machines waiting to land.

 On receipt of the signal "QFG," or "Motors Over," the aircraft is turned on to a heading of about 30 degrees from the reciprocal of the track on which it is intended to make the final approach, and height is reduced to the minimum which is safe in that area. The undercarriage is lowered, the airscrew pitch reduced and as much flap applied as can be used without adversely affecting the subsequent takeoff and climb should the approach have to be abandoned.

The series of QDMs is continued (there is no point in changing to QDRs as sometimes advised—it only leads to confusion), and after, say, two minutes a turn at a rate of about 180 degrees a minute is made until the bearing of the desired approach line is picked up. Frequent bearings are naturally essential, and much depends on the -skill of the radio operator, who should be able to get at least three reliable bearings a minute, although two good bearings are infinitely better than three or more doubtful ones.

Descent is made to the minimum safe height on the approach line selected and speed reduced to the minimum for handling the aircraft with full control.

If the ground becomes visible, even if only vertically beneath, before or when the minimum safe height is reached, the approach will probably be successful provided that correct bearing is maintained.

If the ground is not seen from that minimum height within minutes after completing the 210-degree turn and picking up the approach bearing, any attempt to continue the approach is dangerous. The aircraft should be put into its maximum climb on the best " escape line " (which may or may not be straight ahead) until an obstruction-free height is reached.

With two pilots it is best for the co-pilot to fly the machine up to the point when the aerodrome is crossed on the preliminary approach so that the commander is free to check up all the many things which must be attended to.

A solo pilot must be absolutely sure of himself and adhere to a prearranged plan to avoid having to work out anything new.

The tricky part with very low ceilings is when actual sight of the ground has to be looked for, it being rather difficult both to fly the aircraft in a necessarily very precise manner as regards course and height (the latter being obviously extremely critical) and to keep casting an eye over the side trying to pick up something definite.

The only advance warning that is obtained is a kind of darkening underneath which indicates that the ceiling is about to be broken.

This should become visible only when the minimum safe height has been reached and clear sight of the ground is not obtained at that height, the pilot must not be tempted to come down " j u s t a little bit l o w e r " in the hope of seeing more, but should abandon the approach.

It cannot be too strongly emphasised, however, that this admittedly crude method of blind approach cannot be recommended, and, as the equipment needed to obviate its use except in emergency is now available, there is no justification for its continued use.

 

The " ZZ " system, so called because the final signal to land sent to the aircraft is " ZXZ," X being the last letter of the call sign of the ground station, employs a D /F installation-cum-listening post situated some 400 metres outside the aerodrome boundary on the approach line, the bearing of which is specified.

If it appears that the aircraft, which approaches on a series of QDMs, passes over the listening post in such a way that a landing is possible, the observer there sends the signal ZXZ or YXY, according to whether he is a representative of the company owning the machine or of the airport authority.

If a landing appears impossible the signal J X J is sent. In some cases the " Z Z " signal is not used, but signals of " M W ," '' MNE '' (meaning t h a t motors are audible at the compass point indicated) are sent, and in practice this appears to work just as well within the limits of the system.

The main disadvantage of the basic scheme is that no warning can be given to the approaching aircraft that it is near the aerodrome until its engines are heard.

If at this instant the aircraft radio operator has just pressed his key to '' run for a bearing ' ' there will be a time lag of perhaps 20 seconds before the '' motor signal"' is received by the pilot, and this time may be sufficient for the aircraft to travel so far across the landing area that a direct landing cannot be made from the height at which the machine will be.

At certain Continental aerodromes this difficulty is met by the provision of an additional listening post some few kilometres from the boundary on the approach line and in telephonic communication with the D/F station, and at Heston a '' ZZ ' ' system has been operated in conjunction with the marker beacons of an approach beacon installation.

Experiment is also being made, again on the Continent, with a D/F post located to one side of the approach line in such a way that good cuts can be obtained all the way down the line from about 20 km. distance.

The goniometer at this post is of an automatic type capable of giving remote indications at the aerodrome D/F station, where the milli-ammeter (which is the actual indicator of the bearing of the received signal) is fitted with a long pointer and is mounted on a large-scale map of the approach sector with the pointer spindle on the map position of the D/F post.

In conjunction with plotted lines, both of bearing and of distance from the aerodrome
D/F station, the position of an approaching machine can thus be seen by inspection every time it runs its transmitter, so t h a t bearings and distances can be sent without the usual delay which arises when bearings from an independent collaborating D/F station have to be plotted by hand

Where there is only one listening post the approach proper is best started from a position over the aerodrome, for as a descent to a very low altitude has to be made before the listening post is reached, the pilot must know his distance from the aerodrome to within close limits, and the only way to do this is to work on a time basis from a passage over the aerodrome itself.

At some airports it is possible to get a series of " QGEs " (bearings and distances taken simultaneously), but even the short time lag while t h e necessary plotting is effected is enough to make the use of these QGEs impracticable in most cases as a basis for descent during a direct approach, for on almost all approach lines there are obstructions to be considered and a premature descent to the very low height at which one must pass over the listening post is fraught with danger.

 

Last-Minute Procedure
A general procedure, similar to that suggested for the first approach system described in this article, will be found quite satisfactory, but when the final "motor s i g n a l " is received, be it " Z Z , " " Y Y , " or " M " and a compass point, and provided the last bearing received was correct, descent can be made from the minimum approach line safe height to below the ceiling with the knowledge that the aerodrome is just ahead. If, by reason of the time lag referred to above, this descent results in breaking through too far across the landing area, it will be necessary to go round again, and the pilot must know by previous experiment in fine weather just how far it is possible to continue an overshot landing with any particular machine. Any tail wind, however light, is most misleading in these cases.

It is rarely possible when approaching on a series of bearings to pick up and hold the exact approach line heading, so it is advisable to steady-up on the first bearing received, which is within, say, five degrees of the actual heading. Any attempt at greater precision is almost certain to result in " h u n t i n g " and a spoilt approach, and if the approach becomes erratic for this or any other reason it is most inadvisable to try to straighten it up by large alterations of course when very close to the aerodrome—much better repeat the whole procedure and eliminate the risk of arriving at the boundary in the wrong direction, perhaps heading towards obstructions.

With an additional listening post the decision as to whether it is advisable to commence the approach with a passage over the aerodrome will depend on the nature of the surrounding country, for if it is possible to fly freely in the locality of a height from which a not too rapid descent can be made after passing the advance
listening post, much time will be saved if the approach is based on a QGE and the minimum safe height for the
locality maintained until the advance listening post has been passed. Otherwise, the certainty obtained by starting from over the aerodrome is essential, as, for example, at certain Swiss aerodromes surrounded by mountains.

The signal in common use to indicate that the aircraft's motors have been heard at the advance post is " i i i i . " Granted practice, complete familiarity with the aerodrome and its surroundings, and skilful air and ground radio operators in close co-operation, a pilot can effect blind approaches by this system consistently and safely with ceilings down to as low as 50 metres, and in visibilities as short as 400 metres. Variation of these figures is naturally dependent on the many factors involved, and they apply to daylight operation only. Whilst any " ZZ " system will compare unfavourably with an approach beacon, it must be realised that it has the great advantage that it can be operated as long as the aircraft has two-way medium-wave communication equipment functioning. Even were all aircraft equipped with receivers for landing-beacon signals, a "ZZ " system would provide a valuable stand-by for possible cases of inability for any reason to receive those signals.

The development of approach beacons giving an interlocked " E " and " T " signal on the approach line, combined
with marker beacons on that line to give indication of distance from the landing ground, has simplified the
problem of blind approach enormously, and up to the present this system affords the easiest and safest solution, enabling as it does the making of regular-service landings in visibilities down to 200 metres with ceilings as low as 30 metres, depending always on the suitability of the aerodrome and approaches and the aircraft used. Absolutely blind landings have frequently been made with the aid of these beacons—indeed, they have been made with other less satisfactory systems—but the large element of risk involved places such landings entirely outside the province of regular operation, and the optimistic and misleading statements regarding them so often expressed by pilots with no real experience of fog landing (to say nothing of nonflying
authors of books on radio and the like) cannot be too strongly discounted. Early attempts at providing continuous horizontal guidance included various interlocking signals on medium and high frequencies, using both keyed emission from crossed loops giving overlapping '' figure 8 '' polar diagrams, and combinations of vertical and loop aerials giving a periodically reversed '' cardoid'' diagram, as well as adaptations of the leader cable which has been used for marine work. Effort has long been made also to provide some form of vertical radio guidance. This was done with the leader cable by earthing it at intervals through resistances of various values so that a constant field strength was produced along an inclined path. With the medium frequency beacons a separate high-frequency transmitter emitting a horizontally polarised wave reflected at an angle was tried, and with the various high-frequency beacons use is made both of this scheme and of one in which an aerial system giving on one frequency a pattern of field similar to that of the well-known Lorenz beacon, affording simultaneous horizontal and vertical guidance.

For reasons which are too complex to discuss here, the writer considers that none of these means for p r o v i d i n g vertical guidance is satisfactory (the main difficulties are those of maintaining a stable glide path and the virtual impossibility of matching up the actual glide of different aeroplanes with the radio glide-path, be this either straight or curved) and vertical guidance is in practice obtained from a sensitive altimeter. It is by no means improbable that a satisfactory radio glide-path will eventually be developed, and then the altimeter may be used as a check against the glide-path indicator. The horizontal guidance given, however, leaves little to be desired, although with high-frequency beacons there have been and are many cases of unsatisfactory equi-signal tracks, dead areas, etc., due to local effects of hangars, high ground and so on. At the short distances involved these troubles do not appear to arise with medium-frequency beacons, which, however, are at a disadvantage in Europe because of the shortage of frequencies available. The standard arrangement of approach beacons provides for an equi-signal zone indicating the approach track, the dots and dashes which interlock to form this being disposed to the left and right of it respectively when facing the aerodrome. With most systems the beacon radiates a reciprocal track also, but whilst there are means of eliminating this it does on occasion prove useful, and In most cases is best retained. Distance from the aerodrome is indicated by marker beacons giving a more or less vertical radiation only. The usual arrangement is to place one just outside the aerodrome and one 4 km. away, but as with fast machines this is found to give insufficient advance warning to facilitate the consistently smooth and quickly executed approaches which are essential at any busy airport, it is proposed to add a third marker farther away on the approach fine, and this promises to be a great advantage.


Ultra'Short-Wave Technique
The approach procedure with a beacon will seldom be the same at different aerodromes, and will also vary according to the route of the aircraft and whether it is able to land direct or has to wait a turn. If the aircraft is for any reason on the side of the aerodrome remote from the blind approach sector it is helpful to cross directly over the aerodrome, as this gives an easy basis to work from, and will, of course, verify the reception of the markers, but if the machine is already in the vicinity of the approach sector it is better and quicker to go straight in. The procedure is identical with medium- and high-frequency installations provided that for using the former there is a separate receiver for the pilot so that the watch on the two-way communication frequency need not be broken. If watch has to be broken because only one receiver is provided, there is no real difficulty, but only a necessity for a somewhat more complicated signal routine to obviate the possibility of misunderstanding. It is not generally known that most approach beacons can be switched and keyed by hand so that ordinary signals can be sent, and by this means the ground station can call the aircraft if necessary when two-way watch is broken ; but, naturally, whilst this is being done the normal characteristics of the beacon are in abeyance.
The same considerations as regards minimum safe heights as apply to other approach methods are equally applicable to a beacon approach, and until the advance marker (if any) and the distant marker have been passed the minimum heights appropriate to their respective positions must be observed. When permission to approach on a beacon has been received, a QGE in the usual manner will give a basis from which to start. The aircraft should be flown so that the beacon axis is crossed at a fairly acute angle say, 20 or 30 degrees—otherwise there will be difficulty in aligning the macnine on it in the limited distance and time available. If the directional gyro has been kept carefully checked against the compass it should not now be further altered in any way. As soon as the equi-signal zone is entered the course is changed to the known bearing of the beacon axis, making an approximate allowance for drift, if any.

If drift is present it is of no avail trying to measure or calculate it with any accuracy, for the aeroplane will be descending through a constantly changing wind, and drift correction can only be made according to the guidance given by the beacon. The trickiest case is when a head or tail wind changes direction with height in such a way that the drift becomes reversed during the descent after passing the distant marker, and this, unless it has been anticipated, can give rise to great misgivings as to the functioning of the directional gyros.

Aural Signals Only
If, after allowing for drift, the D.G. readings do not agree with the known beacon axis bearing when the aircraft is kept in the equi-signal zone, it is of no moment provided the apparent error is not more than, say, 10 degrees. The important thing is to stay in the equi-signal zone to the exclusion of all other considerations as far as direction is concerned, and for this much practice is necessary to ensure smoothness. It is better to ignore the beacon receiver visual indicator entirely, for there are quite enough other visual instruments to be watched, and it is manifestly logical to use as many senses as possible so that the minimum number of items are apportioned to each. All the useful information the beacon can give is audible in the earphones,
so that the only instruments to be watched are the directional gyro, the artificial horizon, the A.S.I., and the altimeter. The necessary attention to flaps, undercarriage, etc., as for other approach methods is probably best given before the beacon axis is picked up, so that no more adjustment to them will be needed when concentrating on the actual flying of the aircraft in a necessarily precise manner.

By working on time from the basis of the QGE obtained, the speed of the machine should be reduced to the minimum for safe handling by the time the distant marker is reached (this is where the advantage of an advance marker is apparent), then a descent is made immediately to the minimum safe height for the area between this marker and a point some 200 metres past the beacon transmitter on the far side of the landing area—this is to provide for the case of an overshot approach. Efforts at adjusting the glide so as to produce a uniform rate of descent from the distant marker to the boundary marker are seldom successful, and in the writer's opinion not advisable, for such efforts introduce an added complication of a variable factor into an already critical process. It must be borne in mind that with almost all transport aircraft the speed and height on crossing the boundary
must be just right if a landing is to be possible at all, and any excess of either, very likely if an adjusted glide does not work out properly, will probably mean going round again. Once at the minimum height allowable after passing the distant marker the approach should be continued orr instruments only, without either looking for the ground or descending farther until the boundary marker is heard, when final descent will bring the ground into view—unless, of course, the ceiling has already been broken. It is a good thing, if only for practice, always to continue any approach on instruments, even if the ceiling is broken before reaching the boundary. For the final picking-up of the ground and the landing the provision of the most extensive contact lighting that can be installed is the ideal to be aimed at.

It is a moot point whether contact lighting extending past the boundary marker outside the landing area is justifiable. The writer inclines to think not, for if pilots will rigidly adhere to the minimum safe-height requirements until the boundary marker is reached without trying to look for the ground, there can be no danger of hitting anything, and the final descent below that minimum safe height will take place just outside the boundary where there will be no obstructions—or where there should be none if the aerodrome is properly laid out! In all cases it is vital to realise that if only one of the several factors involved is in error the approach is unsafe, and quickness to perceive such errors is as essential as ability to carry out the actual manoeuvres demanded by the approach itself.

Possibility of Failure
Observance of a system as suggested will cover all eventualities, including failure, for any reason, to pick up the markers, which is a very important consideration. If the distant marker is not heard there will be no descent below a safe height for any position in the vicinity, and provided the beacon axis is followed the passing of the cone of silence of the beacon transmitter will indicate that the landing area has been overshot, when, of course, it is obvious that the approach is a failure unless the ground comes clearly into view. If the boundary marker is not heard, the aerodrome will be crossed at a height which, whilst very low, is still sufficient to clear all obstacles that may be encountered before passage over the cone of silence of the beacon transmitter gives warning that the maximum climb consequent on an overshot approach must be begun.

Failures in service with beacon approach apparatus are very rare, but there is no reason to neglect the most careful provision for all possible sources of trouble that may arise.

There are many incidental points which in all methods of blind approach are found to need attention, and many

minor difficulties which can be overcome only by constant practice. It cannot be too strongly emphasised that if
operation in foggy weather is to be pursued to the utmost possible safe limit it is essential that pilots should be given every opportunity and encouragement to carry out the necessary personal experiment and practice at various aerodromes, and that the question of full-scale training in all methods of blind approach and take-off—a matter lamentably ignored in this country, except in a few cases should be given the consideration it warrants.