Z approach?

[DB6]

In his book 'Air Road to the Isles', Capt E E Fresson refers to a 'Z' approach which they flew in very low cloudbases and vis, but said it was too complex to explain in the book. Anyone know more?
Excellent book by the way :ok:

[JB007]

Try the Questions Forum DB! You may get more of a response - and i'm intrigued too!!!!!

[JW411]

I came across this question donkey's years ago and I have a feeling that the answer was an SBA approach.

[Atcham Tower]

The RAF called it a ZZ approach. I don't know the full details but it involved a controller standing on the watch office/tower roof with a mike and long lead. The aircraft was homed to overhead with DF and the engine sound confirmed this. (Not sure what happened if there was more than one aircraft about :)) It was then given an outbound heading to fly for a fixed number of minutes and then a final approach heading, presumably with descent instructions. Again the engine sound was monitored.

[welkyboy]

I seem to recall this was referred to as a "ZZ" approach. Practiced at Croydon before the war.
Basically the controller listened for the aircraft engine noise overhead and then transmitted ZZ by morse, the pilot then let down hopefully to the airfield, the likelyhood of misidentification was large. This was used before the advent of VHF

[Dan Winterland]

The request for the approach was the transmission of 'QGH'. The aircraft was home to the overhead by RDF, the signal from the ground was transmiited as 'ZZ' to indicate the overhead had been acheived. This was determined by the the DF kit, but as the early DF sets had a poor overhead indication, an audible check was used to confrim it.

Sounds archaic. but I was operating RAF aircraft from an airfield whre the only approach was a QGH as late as 1993! The DF sets were digital by this time though, so the audio check wasn't needed. And we had UHF radios, so no morse!

[DB6]

Thanks chaps. I don't have the book to hand but very low limits were mentioned (about 50 feet cloudbase and 200 yards viz) but this was the 1930s and I don't imagine the Air Ministry had got around to setting approach minima. Also as they had whole fields to land on azimuth tracking wouldn't have been such an issue. Still sounds damned impressive though, especially in the North of Scotland in winter :eek:!
JB, there is no greater mine of information on PPRuNe than AH&N! Oh ye of little faith :ok:.

[aviate1138]

"Quill had shown a natural aptitude for instrument flying and his next posting was to the Meteorological Flight at Duxford, where he became its CO in November 1934. Back on Siskins, he had the misfortune to crash-land on 14 March 1935 in marginal weather conditions, resulting in the almost inevitable 'Siskin nose'."

Quill flew EVERY DAY for a whole year [Given the AFC for so doing] in rain, fog etc. Didn't always land back at Duxford having taken met readings up to about 25,000 feet in sometimes atrocious weather. Then he joined Supermarine as a Test Pilot.

No wonder the Spitfire turned out so well!

[Fareastdriver]

Seem to remember a technique my father told me about to get back in fog.
You were homed onto the airfield so that you approached it on the runway heading and when you got close they fired a ‘porker’, a parachute flare, vertically above the tower. The aircraft then knew where the tower was and carried out a timed circuit including the approach to the runway. Being fog there was no wind or drift.
Apparently you could pick up enough details to land it successfully at about 200 feet.

[DB6]

Not much use in Shetland though, where I've seen fog in 45kt winds!

[Hyperborean]

The "Porker" if I recall correctly was a mortar, usually located adjacent to the signal square. Elderly, to me, colleagues in the late 60s used to trot out the phrase,"Engines overhead, fire the mortar," on occasion.

[Aaaaaaaaaaaaaaaargh!]

Wouldn't you be better off having the bloke with the ears and the microphone stand somewhere other than the tower, like further 'up' the approach path or something. Or did they not have microphones leads long enough for that?

[Fareastdriver]

If somebody is doing a timed circuit the last place you want to be is anywhere near the approach phase.

[Warmtoast]

Just like to add my two pence worth to this thread as although not involved in ZZ approches I was for many years involved in QGH approaches as the following shows.

QGH
As a RAF VHF/DF Operator for over eight years in the 1950’s I provided the bearings for many “QGH’s”. In the 1950’s in the RAF a ‘QGH’ was a request made by a pilot for a ‘Controlled Descent Through Cloud’ and the procedure was to home the aircraft to overhead the airfield with ‘Magnetic Bearings to Steer’ (QDMs).
QGHs would be controlled by the air traffic controller, but on a couple of occasions I did it myself, which was probably against all the rules and regulations, but they worked.
Procedure: The pilot would give a ten-second transmission on the RT which would allow the DF operator to swing the DF aerial to find the ‘null’ on the transmission and by depressing the ‘sense’ plate (which put the aerials out of phase) determine that what was being shown on the DF wheel against the cursor was the correct bearing to the aircraft and it was not a reciprocal. When the sense plate was depressed the signal either when up or down, if it went up the bearing was wrong and the reciprocal was indicated, it was then a matter of swinging the aerial 180-degrees to find the ‘null’ again, go ten degrees either side of the ‘null’ point and depress the sense plate again, this time the signal should go down and if it did one had the correct bearing.
The circumference of the DF wheel was marked with two scales. Top scale showed true bearings from the VHF/DF (QTE’s) whilst the lower scale was marked in red and showed the magnetic course to steer to the airfield (QDMs). To home the aircraft to the airfield overhead for a QGH one read off the bearing shown on the bottom scale, passed it to the controller who in turn passed the magnetic course to steer to the airfield to the aircraft. Aircraft RT transmissions were given every minute or so (or less) with the DF operator taking the bearings. When the aircraft reached the overhead the aircraft’s transmissions sounded all mushy; confirmation that the aircraft was overhead was established by depressing the sense plate and if there was no increase or decrease in signal the aircraft was in fact overhead.
Having informed the controller that the aircraft had reached the overhead, the controller told the aircraft to steer an outbound course about fifteen or twenty degrees to the right of the reciprocal of the inbound runway heading and to descend to an agreed height, possibly 1000ft. The outbound track was flown I seem to recall for about two (or perhaps three minutes). At the end of the two minutes the aircraft was asked to do a rate one turn onto the inbound runway heading, which if all had gone well placed him very near the extended runway centre line at 1000ft. On the inbound leg DF bearings were taken which allowed the controller to check that the aircraft was steering the right course inbound. The controller also gave heights to descend to, so perhaps with one minute to fly to the airfield the aircraft would be at about 500ft and descending to the minimal obstacle height. Unless flying in exceptionally poor visibility the aircraft would see the approach lights and land.
This is all culled from methods last practiced by me over fifty years ago, so if there are any inaccuracies, blame it on age, but the principles are as I remember them.

Photos below show a VHF/DF Station, the radio layout and taking a bearing.


http://i145.photobucket.com/albums/r231/thawes/ThornhillVHFDFHomerExterior.jpg

http://i145.photobucket.com/albums/r231/thawes/ThornhillVHFDFHomerInterior.jpg

http://i145.photobucket.com/albums/r231/thawes/ThornhillAtthewheel.jpg

[John Farley]

Warmtoast

10/10

We also used to do them speechless in Hunters

[Warmtoast]

JF

Thanks. The only part of the procedure that I was hazy about was the timing of the outbound leg. I was also familiar with RT failure let downs (four clicks to let ATC know you had RT trouble - I think - etc).

Manual VHF/DF Homers were gradually phased out during the 1950's in favour of the new Automatic VHF/DFs, which meant the man in the tower controlled the homing and descent from the automatic DF readings from his CRT scope, with no need of a VHF/DF operator to act as middle man.

That being said, manual VHF/DF Homers were still in use until 1958 at various RAF units around the world. Normal practice was for the 'Homer' to be situated in the midddle of the airfield about 100-yards from the main runway. An ideal position when giving bearings to an aircraft on the final approach during a QGH. On the other hand not such an ideal place to be if an aircraft should drift on take-off as the photo below shows!


http://i145.photobucket.com/albums/r231/thawes/ThornhillVHF-DF-After.jpg

The aircraft, a Harvard with a student and instructor aboard during a night overshoot swung wildly wide and clipped its wing on the VHF/DF antenna. The resultant damage (as shown above) wrote off the VHF/DF Homer, the aircraft survived (with a large dent in its wing) and made a hairy but safe landing. The operator on duty was very lucky, he was shocked but unhurt, had the aircraft been a couple of feet lower a major tragedy would almost certainly have been the outcome.

Happy and memorable days!

[Dan Winterland]

We were using the QGH at Swinderby as late as 1992. It was our only instrument procedure and we used it in anger many times. I seem to remember we homed inbound at 2500', went outbound for two minutes descending to the platfrom turn of 1700', then descended to the minima of 760' where we were supposed to be visual and join downwind for a low level circuit. The students used to do them as part of their IF test - on the Turn and Slip and DI only! Considering they had about 40 hours at that point, it was a real acheivement.

Tell that to the kids of today!

[diginagain]

I'll have to check my logbook when I get home to see how many (few, certainly less than 40) hours I had at the time, but ISTR doing a couple as a u/t pilot on the Basic Fixed-wing part of my Army Pilots Course in 1988. It all felt very 1950's, being in a Chippie too, but it achieved the aim.

[Fareastdriver]

Does anybody remember the SBA/TBA approach? They should do because it was used at Ternhill in 1960/61. It was part of our Intermediate Instrument Rating and as students once we had that we were sent off with 8/8 at 1,000ft. and expected to return full or limited panel.
The transmitter was arranged to transmit ‘N’s to the top right and bottom left sectors and ‘A’s to the other two. In the middle was a small cone of silence. At the joins the dit da and the da dit combined to form a continuous tone. This was the beam for runway 23 with two marker beacons that you followed inbound. There was a technique to find the cone of silence yourself but Ternhill Approach gave you a steer. Once in the C of S you went outbound and if the tone changed to ‘N’s you turned right a bit and vise-versa. At the second marker your did a teardrop turn and at now the first marker you started your descent except this time turn left a bit on the ‘N’s and right a bit on the 'A's..
IIRC at 350ft. you levelled and when the inner marker came up you could continue the descent.
Some people with about 10gallons remaining followed the beam down to the runway.