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NEW CODES FOR TAKE-OFF
Hammered out at the I.C.A.O. Airworthiness Committee meeting in Stockholm during July and August 1959 were new aircraft performance requirements that now only await the Organization's full approval before they become the basis for new national airworthiness codes. Among the most significant aspects of the Committee's work has been that on take-off for which a complete new range of terms—already airline pilots' jargon—has been conceived. The following explanation is condensed from the Monthly News Bulletin published by I.F.A.L.P.A.
The take-off case today is largely dominated by the requirement that the aeroplane must leave the ground at 1.15 (piston engine) or 1.2 (jets) times the power-off, free air stall. The essential difference in the case of the new code is that this condition is not required to be met until the aircraft is 35ft above the runway. Thus, between the surface of the runway and 35ft is a segment not protected by a 15-20 per cent margin above the stall. But other protection provided by the new specifications is claimed to be as good, and certainly to be adequate to prevent difficulty in getting airborne or sinking once in the air. But an exact comparison of the old and the new specifications is complex since the datum and the technique applied to take-off have been fundamentally changed.
The important speeds associated with the old take-off were (1) the stall (VSI); (2) the minimum control speed (VMC); (3) the decision or power-failure speed (V1); (4) the take-off safety speed (V2); (5) tlie initial climb speed (V2 or V2+,). There were various connecting links between these speeds but, as indicated above, the one usually dominant was that V2 had to have a 15 per cent (pistons) or a 20 per cent (jets) margin above the stall.
The important speeds associated with the new take-off are: (1) the stall (now Vs); (2) the minimum control speed (VMC); (3) the engine failure recognition speed (V1); (4) the minimum unstick speed (VMU); (5) the rotation speed (VR); (6) the unstick or lift-off speed (VLOF); (7) the minimum take-off safety speed (V2 min) and (8) the initial climb speed (V2). Space does not permit the full description of all these terms but the overriding points to be borne in mind are that lift-of (VLOF) is governed by 12 per cent above minimum unstick (VMU) and is—in the normal case—assisted by ground effect; and V2min (the old V2), with its 20 per cent (jets) margin above the free air, power-off stall, is now not specifically required till 35ft is reached.
The important newcomers are VMU, VR and VLOF and their connection is as follows. Unstick or lift-off and all speeds up to the 35ft height point (at which V2 min is reached) are mainly dominated by VMU. This is a speed used only in certification trials and is the lowest speed at which the aeroplane can be made to leave the ground and climb away safely. This should, of course, be substantially above the stall but it is difficult to relate to the stall because (1) most aircraft gain from ground effect (usually considered to apply up to half wing span height) and it may therefore be possible to unstick at or very near to the stall, which is a free-air power-off figure; and (2) the stall itself, as measured in U.S. certification trials, is not the same as the stall known to pilots and can, in fact, be much less (Comet 4, 5 kt; DC-7, 7 kt; KC-135, 9 kt). Anyhow, the fact remains that a test pilot must demonstrate that, for various weights, the aircraft can be made to unstick at VMU. This figure is then multiplied by 1.12 to give the lift-off speed. This is 'the speed at which the aircraft first becomes air-borne' and is the speed applied in the normal operational take-off.
The rotation speed (VR) is the lowest speed at which the nose gear should leave the ground. However, the rotation speeds currently being applied to jets are based on the American SR-422 code and thus associated with the old V2 (usually VR=V2 less 5-10 kt), while the new rotation speed (which will probably be applied to all jets in the very near future) is not directly related to V2 but to VMU, as described above. It is also related to the stall (1.1 VS) and to the minimum control speed (1.05VMC) but these will not usually be dominant.
Summarizing, once the nose gear has been raised at a speed between VR and VLOF the aircraft should climb away, reaching V2 min speed by 35ft or earlier. On leaving the ground the most likely dominant protection is the 12 per cent above the 'worst leaving condition' (VMU) and at 35ft the dominant protection is approximately the same as we have always had at the old V2, though with perhaps a small loss due to slipstream effect being greater than the 5 per cent allowed for as between the 1.15 VS1 (piston) and 1.2 Vs1 (jet) specification.