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# 1.67 rule

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# 1.67 rule

29th Aug 2010, 10:40

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1.67 rule

I was wondering why the required landing distance has to be multiplied by 1.67 for commercial flights. For sure, it is good to have a safety margin but why couldn't it be 1.25 instead of 1.67?? Where is the origin of this rule, on what performance grounds is it calculated???

Cheers,
Cecco
29th Aug 2010, 10:58

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Hi,

Checkboard posted this reply in Jan.

http://www.pprune.org/questions/4010...-distance.html

From EU-OPS 1.475
General

OPS 1.515
Landing — dry runways

(a) An operator shall ensure that the landing mass of the aeroplane determined in accordance with OPS 1.475(a) for the estimated time of landing at the destination aerodrome and at any alternate aerodrome allows a full stop landing from 50 ft above the threshold:
(1) for turbo-jet powered aeroplanes, within 60 % of the landing distance available; or
(2) for turbo-propeller powered aeroplanes, within 70 % of the landing distance available; ...

same maths - just different ways of looking at the problem.

Cheers RRR.
29th Aug 2010, 11:18

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@rudderrudderrat

why you have to complete the landing within 60% of the landing distance available. Who came up with the 60%, what is the calculation behind this??
Cheers
Cecco
29th Aug 2010, 11:56

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@Cecco,

Who came up with the 60%, what is the calculation behind this??
I've absolutely no idea. I just comply with the legislation.
29th Aug 2010, 12:24

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When you go to your FCOM/QRH and find your Landing Distance Without Auto-Brake, the numbers you get are for (Actual Landing Distance) a 50ft over the threshold, No Flare and Maximum Brake. (I believe this are minimum demonstrated figures).

The 1.60 rule, allows for smoother landings, flare included, low on brakes, and longer touchdowns.
If you compare those Landing Distance Without Auto-Brake tables with the Auto-Landing Distance ones (with auto-brakes medium), you will find that the later ones are roughly the first ones (Actual Landing Distance) multiplied by 1,60.
29th Aug 2010, 12:43

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I don't understand the physics. The idea of landing within 60% for turbojets or 70% for turboprops I believe is to allow for pilot error with respect to speed control and landing technique. It is intended to prevent a slightly high energy approch resulting in an overrun or a prologed float having the same result. Energy varies with respect to the square of the speed but the factoring is a simple factor. So the legislation means that the faster your aircraft ref speed is the smaller margin of error the legislation allows.

Which is wierd. It means the rules allow you to be an untidy pilot if you fly an aircraft with a lower ref speed..

It also means that if you are a reasonable pilot but are landing at an airfield with limiting public transport runway length that what the rules actually do is limit the amount of fuel you can arrive with. Which is not great..

I tried to find out the origin of the change in factor from 1.43 to 1.67 when I was on a postholders course and nobody seemed to know. Its interesting that a number of operators (BA, Virgin etc) seem to use a factor of 1.5... lets play spot the level playing field..
29th Aug 2010, 13:46
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The older UK rules had 1.43 as the standard for alternates, with 1.67 applying to destination. 1.67 has been in vogue for many decades and the origin would have been part of the normal Certification Standards evolutionary process.

A review back through FARs and CARs might give a clue for anyone who is interested.
29th Aug 2010, 18:36

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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.
29th Aug 2010, 20:22

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Apparently there are still about 50 landing overruns a year worldwide. Perhaps we should be using a bit more than 1.67
29th Aug 2010, 23:43

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How much do you want 2.0, 2.5, 3.0?

The rule doesn't cause the over runs. Poor decision making does.
30th Aug 2010, 05:38

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SUVAT

Tommoutrie,

Although the factor seems to be "linear", it is applied to the total distance which already takes the v-squared into account. For example, if you oversimplify landing after WoW to a constant de-acceleration rate of say 'a', then distance to stop = 0.5*(initial velocity)*(initial velocity)* a

On reflection this is reasonable since a large 'a' gets the SLF pushed hard into the seat restraints, and without the help of draggy devices and/or reverse thrust, 'a' cannot exceed 1g. So a "linear" factor allows a more comfortable de-acceleration rate.

Now as to what factor is appropriate .....
30th Aug 2010, 12:14

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Post-war Transport Aircraft

Post-war Transport Aircraft
Dr. Edward P. Warner's Wilbur Wright Memorial Lecture

From page 636 in FLIGHT June 17th, 1943: (Flight International Archive)

It appears that the necessary rate of climb can hardly be attained with a loading product of 500 in a twin-engine aircraft, for example, unless the wing loading is at least 50 lb. per square foot and the take-off power loading less than 10 lb. per horsepower.

Upon the assumptions used herein, the maximum wing loading that is allowable without undue increase in stalling speed becomes a limiting factor in seeking maximum economy of operation at speeds in excess of about 225 m ph. At speeds below that figure it is likely to be the take-off condition that is controlling.

Have we now carried elasticity in regulation, and the adaptation of regulatory requirements to particular operating conditions, too far? Or not yet far enough? Certainly the new code is far more complex than the old. In 1939 the specifications with respect to the landing and take-off performance of aircraft consumed only 16 lines of the American Civil Air Regulations. They now occupy 41/2 pages. Yet it seems impracticable to secure a sufficiently definite statement in less compass, and I believe that there are few if any of the American manufacturers or operators who would forgo the advantages of the flexi-bility of the new regulatory code in order to regain the simplicity of its predecessor.

I believe that we are on the right track in establishing correlation between the characteristics of the aircraft and the characterises of the route on which it is to be operated ; and I believe that we shall see much more of the same sort of thing.

In addition to the provisions relating to the determination of landing distance and take-off distance, the American regulations, unlike those drawn up under the I.C.A.N., provide for maximum limits on stalling speed. The general improvement in the smoothness and firmness of airport surfaces, and the steady increase in mechanical reliability and consequent decrease of the hazard of having to land elsewhere than on a regularly prepared airport surface, have diminished the direct importance of the speed at which the aircraft makes contact with the ground. The speed of approach to a landing, however, is still a serious consideration.

It seems to be the prevailing view among experienced airline pilots in the United States that they do not want to have to make approaches, under present conditions, at over 120 m.p.h. Good practice seems to require also that the speed maintained during the approach should be at least 40 per cent, in excess of the stalling speed. Combining the 120 m.p.h. with the 40 per cent, reserve indicates a stalling speed of 85 m.p.h., and that is the level at which American regulation currently sets the maximum allowable in the "approach condition."

The loss of one engine characteristically reduces the fullpower rate of climb by about "6ooft. per minute in a fourengine transport aircraft and 1,200ft. per minute in a twin-engine one.

There is a further limitation of maximum stalling speed in the landing condition, with full flap, to 80 m.p.h. I will not dwell on that, beyond saying that it becomes critical only for four-engine aircraft with a relatively low power loading.
30th Aug 2010, 12:40

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The 60% and 70% rules are "lines in the sand", along with numerous other "lines in the sand" applying to operations. Why 35ft screen height, and not 30 or 40 feet, or even 10 Metres in our increasingly Metric world.

The 60% rule is a good one to account for the differences between standard procedures for normal (and non-normal) operations by ordinary pilots, versus the techniques used by test pilots in evaluating landing distance. Consider the following sample -

(1) The test pilot approaches the runway crossing the threshold at 50 feet and at Vref. We mortals stick to the 50 ft rule, but carry a minimum of 5Kt additive, or half the wind component if greater. Thus the landing speed will be greater.

(2) The test pilots chops the thrust as he/she crosses the threshold at 50 ft. My gawd!, would we do that? (Typically we mere mortals slowly bring the thrust to idle during the landing flare).

(3) The test pilot aims for the 300M / 1000 ft touchdown point (so do we), but then "plants" the aircraft spot on the markers. We aim for it, and then flare at the appropriate moment, which by all of the rules of physics means that we MUST touch down beyond the 300M / 1000 ft touchdown point.

(4) The test pilot then applys MAXIMUM Braking (the same as in a Rejected Takeoff) to bring the aircraft to a full stop on the runway. Do we want to do that on an every landing basis I think not, the passengers, engineers, and the accountants will not be impressed.

SO - We land faster, keep approach thrust longer, land deeper, and routinely use much less braking than the test pilot, and we normally don't come to a full stop on the runway. We call it normal operations, and the regulatory authorities add a nice buffer to allow for it.

A Line in the Sand? - YES, but a very good one at that.

Regards,

Old Smokey
30th Aug 2010, 13:19

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I seem to recall that during (jet) flight tests under the old BCARs the testing required:

Vat + 15 kts on a wet runway, or
Vat + 15 kts on a dry runway plus an added 15% wet runway factor.
Both conditions required normal retardation, i.e. no reverse!
I believe this created the 1.67 and 1.92 factors over Normal Vat at THR.

The boffins would normally choose the smaller end-result as the Flight Manual starting point. Remember in the UK we always were deemed to land on a wet runway........
Under FARs/JARs the Dry runway option appeared, all aimed at increasing the Max Perf Limited Ldg Wt off critical runway. Some countries did choose FARs as the certification standard for precisely that reason.

IT
30th Aug 2010, 20:03

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How much do you want 2.0, 2.5, 3.0?
No more required, I was just trying to point out that although 1.67 (60%) seems like a large margin it is in fact nothing like as large as many people think. It is eroded by not using max effort braking and often further eroded by poor technique and bad decision making to the point that an overrun occurs. Reverse should in theory give you extra margin but if used in conjunction with autobrake won't normally decrease your stopping distance.

The bottom line is that if you land on a runway at the max regulated weight you will be very unlikely to have 40% of the runway still in front of you by the time you stop.
31st Aug 2010, 01:44

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Cecco re #1 & #3 ”why you have to complete the landing within 60% of the landing distance available.”
EU Ops only requires that the landing is planned to stop within 60% of the distance available. You probably appreciate that you don’t have to complete a landing in that distance; although for contaminated operations you might want too – see below.

EU Ops also requires that pilots assess the conditions in-flight to ensure a safe landing; this requires a safety factor to be applied; this is as for pre-flight or similar.
”This factor accounts for the normal operational variability that can be expected in day to day service such that the chances of a landing overrun are remote.” Landing performance of large transport aeroplanes, UK AIC 14/06.
AFAIR the definition of ‘remote’ in this instance comes from certification criteria and historical aspects as in previous posts.

Max Angle #17 ”… although 1.67 (60%) seems like a large margin it is in fact nothing like as large as many people think. It is eroded by …
Absolutely correct; but re “no more required” might depend on the circumstances. *1

Old Smokey, with respect (OB1), the certification landing tests are not quite as you describe (#15); re landing aim point and thrust management, and the need to use max brake in operations.
The key issues are that the unfactored distance is most unlikely to be achievable, and it is used to define ‘a line in the sand’; but the problem is that the sand is shifting!

Where factored performance data provides a landing distance safety factor – a safety margin, the margin varies with the situation. Thus a 1.92 wet factor might provide a safety margin equivalent to the dry safety margin in ‘good’ wet conditions, the actual safety margin can reduce rapidly with increasing ‘wetness’ to a point where no margin may exist in wet contaminated conditions.
The safety margin can also be eroded much quicker with changes in tyre condition, type of runway surface, and runway maintenance condition; thus even with a ‘legal’ LDR the safety of the operation requires pilot intervention to avoid or mitigate the actual conditions. *2

Furthermore, in contaminated conditions there may be no safety margin (even with a 1.15 factor) due to the way in which performance data is acquired and the assumptions made about the runway conditions - conditions which in general the pilot is poorly informed of. *3.
Thus “Attempts to land on contaminated runways involve considerable risk and should be avoided whenever possible” (UK AIC 14/06).

Therefore, the pre landing assessment requires knowledge of how far the sand has drifted, and on what basis the line is drawn – drawn more often with a wide brush rather than a fine pencil.

* Refs
(1) “The current operational dispatch factor of 1.92 for turbojet aircraft landing on wet runways at destination or alternate airports would have to be increased to a value of 2.2 to 2.4 in order to achieve the same level of safety as that which is currently accepted for dry runway operations. Transport Canada TP 14273E.

(2) “…using the braking coefficients obtained during the tests on wet surfaces, indicates that the current operational dispatch factor of 1.92 for turbojet aircraft does not provide an adequate safety margin for landings on wet runways, particularly those with low texture or rubber contamination”. Transport Canada TP 14627E.

(3) See presentations at EASA - Workshop Runway Friction and Aircraft Braking.
31st Aug 2010, 07:53

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Does 1.67 has something to do with the length of the touch-down zone perhaps?
31st Aug 2010, 08:07

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Originally Posted by wmuflyuy
The rule doesn't cause the over runs. Poor decision making does.
To repeat two mantras which most of us who analyse accidents have been trying to promulgate for decades:
1. Accidents generally don't have one cause. They have many causal factors.
2. Putting the responsibility on the people in the pointy end does not necessarily lead to effective countermeasures.

Amongst causal factors which have arisen more than once in noteworthy overrun accidents have been
* misleading weather reports, in particular wind (for example, pilots are expecting a headwind on the runway and they encounter instead a tail wind)
* state of the runway surface (particularly low coefficient of friction; presence of a disadvantageous amount of standing water)
* misleading reports, or lack of reports, to flight crew of runway friction characteristics
* airline SOPs (for example, how many extra knots to be carried/may be carried on approach, under what conditions and under what expectations)
* inappropriate assumptions by procedure designers about how pilots behave upon experiencing a lack of expected braking performance
* inappropriate decisions made by flight crew

There have been overrun accidents in which, I would argue, the flight crew's decisions were appropriate in light of the information they were given, the operating procedures of their airline and the understood characteristics of the aircraft they were flying. (But, of course, not appropriate according to the reality!)

PBL
31st Aug 2010, 08:16

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1,67

Perhaps it is simple mathematic rule in order to get 60% out of 100% you must multiply the L50(Landing Distance) by 1,67 in order to get the Landing Field Lenght.
You can try and check let's suppose a plane lands in 1800 metres having 1800X1,67=3006 so Landing Distance 1800m Landing Field Lenght 3000
now if you make the 60% of 3000 it comes exactly 1800.
When a pilot have to check the maximum landin weight has to go back fron the available runway , take the 60% and find at wich weight a plane will have a landing distance long that amount.
that will be the maximum landin weight limited by the runway lenght that have to be compared by the landing climb limited weight and the structural weight , the lowest will be the Maximum Landing Weight MLW.
cuvcap
31st Aug 2010, 13:10

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