1.67
The work of the ICAO AIRWORTHINESS COMMITTEE might shed some light on the subject. The Third Meeting of the Committee was held in Stockholm 14 July to 10 August 1959. At this meeting the Committee prepared the ICAO CIRCULAR 58 – PROVICIONAL ACCEPTABLE MEANS OF COMPLIANCE – AEROPLANE PERFORMANCE
From the report of the Third Meeting:
1.2 LANDING DISTANCE PERFORMANCE REQUIREMENTS
Introduction
1.2.1 At the Second Meeting the Committee recognized that the landing distance reqirements that were incorporated into the draft PAMC on Performance were deficient in some respects, and that improved landing specifications should be developed. Mr J.D. Harris was assigned the preparation of a working paper indicating the work done on this subject in the United Kingdom.
1.2.2 At this meeting, the Committee had before it the working paper presented by Mr. Harris, and prepared by Mr. L.J.W. Hall, and also comments on the whole problem of landing distances, presented by Captain Miles.
1.2.3 Much of the criticism of the landing distance requirements which was expressed when it was decided to incorporate them into a PAMC is concerned with the factors relating the required distance to the distance measured in certification tests. At the Second Meeting, however, it was felt that the problem was not merely to determine coefficients suitable for each kind of aeroplane (propeller driven, jet, with or without reverse thrust, with or without air brakes, etc.) and for each set of operating conditions (destination or alternate, dry or wet runway, etc.), butt hat it was also necessary to re-examine the whole of the specifications. The fact that the margins are large, providing for field lenghts much longer than the distances measured in certification, indicates that there is a wide area of uncertainty as to what variables the margins cater for, and therefore doubt is cast on the adequacy of the requirements themselves. This doubt is increased by the fact that there have been numerous incidents at landing and these have recently led two of the States which have designated members to the Committee to increase the coefficient from 1.67 to 1.82 for turbo-jet aeroplanes not fitted with thrust reversers. In analyzing the deficiencies of the landing distance requirements, the Committee came to the conclusion that the specifications did not provide for a uniform level of safety for the reasons given in 1.2.3.1 to 1.2.3.9 inclusive.
1.2.3.1 According to para. 9.2 of Part I of the PAMC, the landing should be preceded by a steady gliding approach down to the 15 metres (50 feet) height point with a calibrated airspeed of not less than 1.3 Vs. Some aeroplanes have approach speeds appreciably above 1.3 Vs and the fact that the specifications in the PAMC fail to account for these differences has caused variations in the achieved level of safety. In operation the speed selected not only gives a suitable margin above the stalling speed, but also provides for sufficient speed stability, lateral control to cater for the loss of an engine, and sufficient lonitudional stability; these factors have frequently resulted in speeds greater than 1.3 Vs.
1.2.3.2 In conditions of limiting visibility and cloud base it is not possible operationally to achieve the flap configuration, the power, and the angle of descent, assumed in the landing specifications in the PAMC.
1.2.3.3 An approach speed higher than that at which the test measurements are made results in a longer landing distance, and to cater for the consequences of such speed increases, large margins were applied to the measured distance. However, the sensitivity to overspeeding on the apporach varies considerably between aeroplanes, and this is not taken into account in the specifications in the PAMC.
1.2.3.4 All operational landings are not conducted on smooth, dry, hard surfaces. These specifications in the PAMC have not taken account of the fact that aeroplanes are not equally sensitive to variations in the coefficient of friction.
1.2.3.5 In recent years, many piston engined aeroplanes were fitted with reverse thrust and, as is mentioned in the PAMC (Note under 9.6 of Part I), national aurhorities have been conservative in giving credit for these devices. The use of reverse thrust, when no credit was given for it, has hidden the fact that the requirementas were inadequate, and consequently, aeroplanes not fitted with such devices have had inadequate landing distances.
1.2.3.6 Although 9.3 of Part I of the PAMC provides that the distance should be determined in such a manner that its reproduction does not require exceptional skill or alertness on the part of the pilot, there are serious reasons to doubt that this is always the case. The technique used in operation are so different from those used in certification tests that it is difficult to determine whether this requirement has been applied with uniform conservatism in the various States.
1.2.3.7 Most of the aforementioned deficiencies are well known to the pilots and have caused a lack of confidence in the regulations. Not being sufficiently certain that they will make a safe landing leads them to decide to land in a manner they consider safer than the one established in accordance with the requirements, a practice which may have resulted in some of the incidents which have occured in the past. All the implications are difficult to evaluate, but it is considered to have been the cause of a number of undershoot incidents.
1.2.3.8 The specifications in the PAMC do not account for the direct effects of crosswind on performance. In crosswinds there is, for example, a tendency to approach at a higher speed.
1.2.3.9 Many of the variables that could be accounted for directly are covered by a large operational margin. This has made it difficult to determine the adequacy of the existing specifications for any given set of adverse conditions.
From the forword of the ICAO CIRCULAR 58:
10. At the Third Meeting, it appeared that several refinements of the specifications in the draft had to be made, but that most of the numerical values established at the Second Meeting were as sound as they could be in the light of technical knowledge and of operational experience obtained with turbine -powered transport aeroplanes. It was recognized that further knowledge, increased experience, or introduction of new types or new methods of operation in the future may necessitate some adjustment, and the Airworthiness Committee decided to bring the PAMC up-to-date as the need arises. The decision to keep the PAMC under review should not be taken as reflecting a lack of confidence in the specifications, but rather as a wish to keep abreast with the technological developments of a technique so rapidly changing as civil aviation.
However, in the particular area of landing distances , the Airworthiness Committee has not yet established specifications in which it has the same degree of confidence with respect to their applicability to the full range of aeroplanes to which the PAMC is intended to apply during the next few years, as it has in the specifications covering other stages of flight. It has made plans, therefore, for further development of landing distance specifications to be included in the PAMC as soon as they can be developed.
Last edited by tribo; 29th Aug 2010 at 20:07.