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Thread: ZZ landings

  1. #1
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    Default ZZ landings

    Hi,

    According to many logbooks, we often read ZZ landings, ZZ approach or ZZ alone in the duties.

    What is a ZZ ?

    TIA

    Bertrand H

  2. #2
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    Using the (Lorenz) or other radio beam due to bad visibility/weather

    http://www.bbc.co.uk/ww2peopleswar/stories/87/a8979187.shtml

    A
    Last edited by Amrit; 28th January 2008 at 18:52. Reason: forgot link

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    Z(ero) visibility and Z(ero) cloud base; in other words, fog and cloud base 'on the deck'.

    Brian

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    Thanks both ! Precise and clear, not like the fog.

    Bertrand

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    Default ZZ Landing System

    Radio Navigation Systems

    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

  6. #6
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    There are some contemporary articles in the Flight magazine archvies about it, secondary sources.

    Search per year

    An example:
    http://www.flightglobal.com/pdfarchive/view/1939/1939%20-%201356.html

    www.flightglobal.com/pdfarchive

    http://www.flightglobal.com/pdfarchive/view/1955/1955%20-%201087.html

    Not mentioned in Fligth magazine from 1944 onwards, only breifly or where the scanner has misread letters!
    Last edited by dennis_burke; 19th November 2009 at 10:44.
    Dennis Burke
    - Dublin

    Foreign Aircrew and Aircraft Ireland 1939-1945
    www.ww2irishaviation.com

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    Default Thank you but how do you...?

    The link below gives excellent information on page 461 of the article but how do you view the next page 462 on these archives ?

    http://www.flightglobal.com/pdfarchive/view/1955/1955%20-%201087.html

  8. #8
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    http://www.flightglobal.com/pdfarchive/view/1955/1955%20-%201087.html


    Type the next consequtive page number in:
    http://www.flightglobal.com/pdfarchive/view/1955/1955%20-%201088.html

    etc, also , is there not a side menu on the left hand side showing thumbnails of the pages, simply click on the thumbnail of the next page, the page your on is highlighted with a blue outline.

    This is the one your on about

    http://www.flightglobal.com/pdfarchive/view/1939/1939%20-%201356.html
    http://www.flightglobal.com/pdfarchive/view/1939/1939%20-%201357.html
    http://www.flightglobal.com/pdfarchive/view/1939/1939%20-%201358.html
    http://www.flightglobal.com/pdfarchive/view/1939/1939%20-%201359.html
    Dennis Burke
    - Dublin

    Foreign Aircrew and Aircraft Ireland 1939-1945
    www.ww2irishaviation.com

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    Default Thank You

    Your detective work is much appreciated Dennis.

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