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Thread: Blind Approach Training (BAT) course

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    Default Blind Approach Training (BAT) course

    Hi
    Not being a techo-wizard I am struggling to work out how a Blind/Beam approach worked in 1943.
    As far as I can understand a fixed narrow radio beam gave out a continuous tone signal when the aircraft was correctly in line with the runway, dots and dashes were heard on either side when flying off line. The pilot had to keep the aircraft flying along this line of continuous tone until he reached the airfield and was able to land.
    Is this anything like a modern radar system?

    Cheers Motherbird

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    Actually two beams were transmitted one composed of dots and the other of dashes. Where they intersected the pilot would hear a continuous tone.

    I believe the same principle is still used but the signals are 'read' automatically and displayed on the ILS instruments or fed into the auto pilot.

    Malcolm

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    Hi
    Just to tidy up, there were 2 transmissions originating from the same point and shaped by the aerials to be elongated lobes. By arranging that they overlapped, a beam was created that could be aligned along a single bearing,commonly a Runway centreline.In the lobe on one side was transmitted continuous "N"s in morse i.e _ . and in the other Morse"A"s . _ with the spaces in between being filled where they overlapped to produce a continuous tone. The pilot would hear this in his headphones and could fly to intercept the continuous signal.There were variations that substituted needle pointers in the cockpit to make a visual indication. The last installation that I am aware of in the RAF was an aural signal and we used it at Ternhill in 1961 until the move of 6 FTS to Acklington. As Malcolm says the current use is with much higher frequencies for the ILS Systems at modern Airports including transmitting an extra vertical signal to provide Glidepath guidance using the same principle, the whole being read off the Flight Management System in the a/c usually via an addition to the Artificial Horizon.Along the beam there would be beacons with a controlled Fan Beam pointing upwards to give some "range from airfield" indication and assist the pilot in his descent profile
    Regards
    Dick

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    Just a small addition:

    The A and N signal described also formed the basis of the commercial bad weather flying network in North America from the early 1930s, with the last stations being deactivated in Alaska and the Canadian north only a decade or two ago. Airways, the offical bad weather and night routes between airports, were defined by a chain of transmitting stations, aligned such that the overlapping, continuous tone was heard when headed directly towards the next station in the chain, or the airport at the end of the airway. Busy stations would have one or even two cross signals, intersections if you like. When my father was a flight instructor in the 1950s and 1960s, the technique of "riding the beams" formed a large part of the advanced training. Elaborate procedures existed for determining if you were headed towards or away from a station, and which of several intersecting beams you had acquired. Each station broadcast a Morse code station ID on a seperate frequency. By 1950s the radios were automated to the point where if you received the station ID, a simple flick of a switch gave you the A or N in your headset from that station.

    The beacon closest to your destination airport would often have a simplifed bad weather approach procedure published, consisting of overflying the beacon (indicated by a brief loss of the steady tone as you flew through the cone of silence directly above the transmitter) at a defined starting altitude, then flying a defined heading, at a defined descent rate. In the simplest systems, this allowed you to descend to some minimum safe altitude without worrying about hills, chimneys, etc. getting in the way, until you broke out of the clouds and could see the ground. Later, more sophisticated approach methods would use this technique to align you with another, more precise, short range navigation aid like an ILS or radar controlled approach.

    The overlapping constant tone area was not just on a single line, but on a pie shaped sector that increased in width as you got further away from the station. It could be tens of miles wide at extreme distances. It could all be a little inprecise by modern standards, but it was all they had back then.

    Experienced pilots, by the way, could collect information on the weather, distance from the station, distance from the steady tone sectors, etc. from the minor changes in the sounds of the A and N tones or the steady overlapping tone in their headsets. It was as much art as science.

    Common causes of accidents with this system included mis-identifying the station, mis-identifying which quadrant of the intersecting beams you were in, and mis-identifying if you were flying towards or away from the station. All these resulted in you being somewhere you didn't think you were, leading to fuel exhaustion or flight into rising terrain.

    Modern ILS uses a phase shift spread across the vertical and horizontal signals, so the instrument can detect not just if you are in the A or N direction away from the ideal path, but how far away you are. All these calculations are performed for the pilot, and presented as vertical and horizontal deviation needles (or computorized replicas) on an instrument in front of the pilot. When the needles form a perfect 90 degree cross in the middle of the instrument face, you are "on the beam".

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    Thanks for info.
    I think I will have to print out your replies to make sure I understand it fully!
    Bill you said that "accidents were caused by mis-identifying the station or flying the wrong way," ie away from rather than towards the station.
    How far away and at what height could the beams be detected?
    David GH on his course flew to Watchfield, Thrxuton, Netheravon and Upavon.
    As Neveravon and Upavon are only about 4 miles apart how could he know which one he was flying to or from?

    Cheers
    Motherbird

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    The installation that I used was specific to assisting approaches to an airfield and as such the area of overlap could be tailored to be a narrow(but far from precise)cigar shape lined up with the runway.What Bill has added is the use of the same principle to provide en-route guidance towards and between airfields and that was much more complex but was often the only alternative to not flying at all over some of the sparser population areas of N America with their often difficult terrain. I am glad that I only had to use the Ternhill airfield SBA. Later when I went into Civil ATC we used to have some amusement at the implications of an old Separation Technique where a/c in opposite directions were told to "Fly well left of the en-route signal". Just as ,in my 70's I wonder how my modern successors would have coped with the systems that we used so I must wonder how I would have dealt with the system outlined by Bill
    As Bill has pointed out each installation would have an i/d signal alongside or superimposed
    Regards.
    Dick

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    As Dick pointed out, each beacon would have a unique code, usually 2 or 3 letters, transmitted in Morse. Also, in North America there were several frequencies available for the stations, ones close together would not use the same frequency.

    Still, humans being humans, people would sometimes misread the ID code, or not set the right frequency, or whatever, and merrily fly towards point A when they thought they were going to point B. This could result in running out of gas, or what we now call "controlled flight into terrain".

    There is a bit more of a write up, and a simple sketch, at http://www.navfltsm.addr.com/ndb-nav-history.htm

    An approach aid, as used in the UK at the time, would only have one of the four "on course" segments or beams shown in the sketch. North American airways could have one to 4 of these segments, as needed. Directional antennas, and blocking of part of the antennas signals, were used to point the beams in the right direction, and "turn off" the un-needed segments.

    The distance over which the beam was useful would depend largely on the strength of the transmitter. Approach aids would have ranges of 10 miles or less, en-route radio ranges could be 100 to 200 miles apart.

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    There is a good description of the Standard Beam Approach system in the accident report for Dakota G-AHCY with a few diagrams to show how it works.

    Basically the two directional transimitters had a 'field of view' of 182 degrees.

    Say an airfield has its main runway orientated N-S (000-180) the two transmitters with be 359-181 (transmitting east) / 001-179 (transmitting west) giving a 2 degree overlap (359-001 and 179-181) with the Western sector being N and the Eastern sector being A. It is easier with a diagram, but you would fly on one side of the beam until the beam went to continuous tone and then carry out a near 180 turn to intercept the beam agains at safe altitude (the time it takes to cross the beam gives a rough distance from the airfield) and follow it to the 'cone of silence' directly overhead the transmitter. Flying over the cone of silence and out along one of the beams allows you to then carry out a further procedure turn to again intercept the beam at a 'given' distance from the field and descend along it to land, in theory.

    If you want I can e-mail the report to you (send me a PM) as it makes a lot more sense with diagrams. It makes up 4 pages out of 23 so a bit short for learning the whole concept but it gives the general idea.
    Alan Clark

    Peak District Air Accident Research

    http://www.peakdistrictaircrashes.co.uk/

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