FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1946
1946 - 1358.PDF
48 FLIGHT JULY IITH, 1946 APPROACH BY GROUND CONTROL craft can be handled, and the Directors who work from them use their own R/T channel which is distinct from the Controller's. Thus one aircraft may be making an approach, while another is being brought into position to do so, and yet others are orbiting while awaiting their turns. Up to fifteen aircraft have been handled simul- taneously in this way, and have been landed at the rate of one every 3-4 minutes. These eleborate facilities were provided for bomber operations, and will scarcely be needed on the same scale for scheduled civil flying, but the continuous control of aircraft in the circuit is a safety factor of great importance. Apart from trie actual land- ing system, the P.P,I. is invaluable for airfield control in conditions of poor visibility. Final Approach The P.P.I. Director has finished his job when the air- craft is about seven miles down-wind from the airfield, within two thousand feet of the approach track and head- ing towards the runway at the correct height (usually 1,500 feet). At this stage the aircraft becomes visible on the radar screens ol the approach or "precision" system, and, as these displays are of a novel type, they will be considered in some detail. In addition to the P.P.I, aerial, which scans continuously through 360 degrees, the G.C.A. has two other radar aerials which produce extremely tight beams—as narrow, in fact, as those of an ordinary searchlight. One of these scans rapidly in the vertical plane through an angle of 7 degrees exploring, as it does so, a narrow triangle of space ten miles long and 7 degrees high. Any echoes picked up are displayed on a pair of "'Elevation 'scopes" on which the ground is shown as a horizontal line and aircraft can be watched flying above it just as if one were standing several A general view of the interior of the trailer showing the positions of the operating personnel. In the foreground is the approach controller's position, and at the far end is the traffic director's post. A close-up of a traffic director's position and his "scope." miles away with a "side view" of the proceedings. The picture is extremely realistic, but is exaggerated about ten times in the vertical direction so that the slightest change in the height of the aircraft can be seen at once. There are two elevation pictures, one ten miles "long" and the other giving an expanded view of the last three miles of the approach. On this "scope" it is sometimes possible to watch the aircraft taxying along the runway after it had landed. An operator, known as the "Elevation Tracker," sits in front of these two screens and, by means of a hand- wheel, keeps an illuminated cursor centred on the aircraft echo. By so doing he feeds elevation information con- tinuously to the Controller, who has a large meter showing him at a glance the aircraft's distance above or below the glide-path. This metei can be read to within twenty feet —hence the phrase "precision system." Save for the last stages of the approach, the Controller is not concerned with the aircraft's actual height, but only with its "error" in relation to a preselected glide-path (usually 3! degrees). Horizontal Precision Azimuth information is obtained in a very similar manner by the second " precision " beam, which scans hori- zontally through a sector twenty degrees wide and ten miles long It will now be seen why P.P.I, control is necessary to put the aircraft into this somewhat restricted region. The resultant echoes are displayed on a pair of "azimuth scopes" which show this triangular sector expanded three times in width, and give the impression of "looking down" on the approach from above. The slightest movement of the aircraft to the right or left is clearly visible, and an "Azimuth Tracker" keeps his cursor on the moving echo as it flies towards the runway. Thus he, too, feeds information to the Controller, whose azimuth meter shows the deviation of the aircraft from the required track. This meter can be read to within an
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
