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Aviation History
1960
1960 - 0865.PDF
FLIGHT, 24 June 1960 873 AIRCRAFT PARKING DIRECTION 5 ft BETWEEN/ 2O5ft STAND CENTRE TO CENTRE KEY TO RADIIComet (solid line) ft R1 =42.5 = 43.5 R3 = 68.5 = 69.5 Caravelle/D.H.121(broken line) ft= 36.5 CENTRELINE Fig 2. Possible apron layout for Comet 4B, Caravelle, D.H.121, etc., based on a 45° nose-out parking angle. The Caravelle guideline as recommended by Air France, could also be used by the D.H.121. Scale thickness of guidelines has been slightly exaggerated for reproduction purposes manoeuvring axis, which is the axis of the mainwheels. On the other hand, when allowance is made for jet core intermingling, which is greater in the case of the Comet, the score is again almost even. From consideration of these points it is not surprising to find that the shutting down of any engines during ground manoeuvring, or the use of asymmetric thrust, is bound to result in the generation of unnecessary extra blast. Experiments to test the effects of apron slope on breakaway blast show that this factor is also important. For example, during trials carried out by BEA, when a Comet 4B was made to break- away both up and down a slope of 1 : 150, it was found that the uphill case required 1^ times the thrust needed in the downhill case. However, a downhill slope for departures is an uphill slope for arrivals, so that the useful application of this factor is rather limited. There are also other reasons for requiring aprons to be as level as is consistent with adequate rainwater drainage. The effect of surface wind is considerable, and the aircraft's blast field can be considerably displaced by cross-winds. As far as head- or tail-winds are concerned a reasonably accurate answer results from the linear addition or subtraction of such a component. Since it is the power applied by the pilot that directly affects the amount of blast produced, his taxying technique is of the utmost importance. The gentlest breakaway will be achieved by centralizing the nosewheel, setting up a pre-calculated minimum adequate power, and waiting for the aircraft to roll. As far as the taxi-in to the stand is concerned, it has been found that with practice good use can be made of momentum; in this respect a well laid out apron is of the greatest assistance to both pilot and marshaller. Acceptable Blast Must we fence each aircraft around with expensive blast-screens? Or, again, must we accept towing with all its delays and operational complications? Let us hope not, for the burdens_ of such solutions would fall heavily on airlines and administrations alike. What blast levels, then, are acceptable? Over a year ago the (then) MTCA carried out a most interesting trial with test "passengers" who were subjected to the blast of a Comet at varying speeds. When the velocity exceeded 25kt many showed signs of distinct discomfort, in spite of the fact that these same people thought nothing of walking home in a much stronger "natural" wind. The psychological effect of standing behind a large aeroplane, in the unfamiliar environment of a warm, paraffin-laden breeze, made all the difference. Any airline wishing to learn from this test would therefore exercise the most watchful care to ensure that its passengers never met such conditions. This would be particularly easy to achieve with a pier system, where outdoor walking distances are low, and where all passenger movement during boarding or disembarking can be controlled from pier gate or aircraft door. Apron personnel can be expected to accept higher speeds than passengers, and it would appear that 45kt can be withstood with- out difficulty. The items of equipment used on the apron have differing blast limits, varying from servicing steps at the lower end of the scale to bowsers and ground-power units at the top. However, it has been found quite a simple matter to anchor the lighter pieces of equipment, while the heavier items cause no problems and may even provide some localized shelter. Small blast screens near the pier, in the "dead area" between adjoining stands (the area which is not overlapped by the wing-tips of arriving or departing aircraft) would answer any remaining require- ment for shelter, without having any effect on stand dimensions. It would not be necessary to apply a blast limit to terminal buildings or piers, since any building should be able to stand a wind of gale force. The maximum blast speed which can safely be withstood by a parked aircraft of Viscount size appears to be at least 55kt. In practice such a figure will rarely be experienced and it is likely that the critical factor in deciding stand dimensions will be wingtip clearance and not the blast limit. Conclusions For many reasons, which are outside the scope of this article, the most efficient apron layout for the short-haul operator is the pier system. There is now considerable evidence to show that with such a layout jet aircraft can and should be manoeuvred close up to the pier under their own power, provided appropriate precautions are taken. In fact, if both apron occupancy and costs are to be kept to a minimum it is suggested that towing must have no part in an operational turnround. If the requirement for more concrete on the apron is also to be minimized the large "three sides of a box" type of blast screen is similarly unacceptable, since it will affect wing-tip clearances and call for a subsequent increase in stand size. This is quite apart from the cost of the screens themselves. The precautions to be observed should cover both planning and operational use of the apron, as well as engine handling technique. They can be subdivided into: — (a) Safeguarding the adjoining "downstream" stand at breakawayby parking aircraft at a nose-out angle of at least 45°. This will allow breakaway blast to impinge on the building rather than on anotheraircraft, and will also allow the "live" aircraft's departure roll to be in a straight line (with resultant reduction in thrust required). (b) Ensuring that stand dimensions are based on a practical opera-tional turning circle with the maximum of visual guidance, a reason- able clearance (25ft) between the outer wingtip of the moving aircraftand other aircraft and obstructions, and the acceptable blast limits of the downstream parked aircraft. For the Comet 4B all these require-ments are satisfied by a stand centre-to-centre distance of 205ft. Estimates suggest a rather lower figure for the D.H.121, but muchdepends on the parking angle involved and the requirements of other types of aircraft on adjoining stands. (c) Safeguarding of passengers, apron personnel and equipment bythe application of carefully developed procedures based on acceptable limits of blast velocity. Although such procedures may involve atemporary curtailment of some turnround activity when a jet taxies into an adjoining upstream stand this solution is far more acceptablethan other proposed solutions. (d) Making use of taxying techniques designed to keep blastvelocities to a minimum: when taxying out, by using a minimum pre-calculated power symmetrically on all engines and then waitingfor the aircraft to roll rather than applying a burst of high power; when taxying in, by avoiding sharp turns, making maximum use ofmomentum during the final turn on to the stand, and once again by using symmetrical power on all engines. This type of taxying tech-nique is in use by BEA's Comet pilots.
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