Landing Field Length

[renzo.zubi]

Hallo! I would like to know why the "Landing Field Length" of a Part 25-certified jet like the Falcon 7x is not influenced by the OUTSIDE TEMPERATURE.

There is a huge difference in TAS if you fly your approach with a defined indicated approach speed (Vref+...) in hot or cold weather. The resulting field length would be quite different.

I'm aware that the Landing and Approach Climb requirements are a function of the OAT/TAT but that's not the question.

Does anybody have an idea???

[DeafOldFart]

maybe I am being a little naive here.... but the answer might be somewhere at the back of the manual.... if it isn't, I'm not buying one of those....

[Mad (Flt) Scientist]

Simplistic/legalistic answer: because it's not required!

§25.125 Landing.

(a) The horizontal distance necessary to land and to come to a complete stop (or to a speed of approximately 3 knots for water landings) from a point 50 feet above the landing surface must be determined (for standard temperatures, at each weight, altitude, and wind within the operational limits established by the applicant for the airplane):

It's DEFINED at the standard temperature. So it's only presented at that one temperature.

That's not an answer to "how does the landing distance in practice vary with temp?" which is a physics question. But LFL (and ALD) are defined quantities, and if the definition says not to consider temperature, then you don't.

(Just like VMC is not, despite the name, the "minimum control speed" - it's the minimum control speed for a precisely defined set of conditions. An aircraft, OEI, can definitely be "controlled" at a lower speed, by allowing the conditions to change. (It can also be uncontrollable at the same speed, if we make the conditions more adverse). But it's defined as a specific set of conditions, and everyone uses that same "level playing field" for consistency.)

[Basil]

Simple pilot answer is that the basic figure is increased by about 60% for the graphs we use plus another 15% for wet.
I think that a temp difference of from -10C to +30C would make about a 20% difference in landing distance required so any imaginable temp* deviation is taken care of by the safety factor.
I do stand to be corrected ;)

* Not some :mad: landing high and fast on a wet runway!

[No Fly Zone]

@MFT nailed it. Implied but not stated is the underlying rule that every PIC, from the AN-225 to a one-seat home built, is personally responsible for the safety of every flight. The endless numbers about weather, visibility, performance, runway length and stopping distance are little more than informational guides for the PIC. I guess we'd all like to see all pilots use all of the available tools to fly professionally in every case, but that cannot happen. Even if it means cancelling a flight or two - or just staying home a few times, please resolve to improve your personal flight safety practices during 2015. If, come 12-31-2015, we are still reading and responding to questions about braking distance, the quirks of brand-specific FMCs and fuel consumption, I guess we will have done something right. Happy New Year flyers and please, fly safe...:D:D

[PEI_3721]

Basil, beware using ‘about’ this or that, a technical professional forum. Also, do not assume that safety factors will take care of uncorrected data without evidence.

The term Landing Field Length (#1) is more often associated with manufacturers’ operational data in the QRH for use ‘in-flight’.

The landing distances associated with factoring originate from the certificated values in the AFM. As per MFS these distances are not corrected for temp.
The AFM ‘actual’ distances almost certainly cannot be achieved in routine operations.
Operational factors (i.e. EU-OPS) are applied to the AFM values for dispatch, but the resulting distances involve many assumptions, predominantly variation in flight path control, speed, and some environmental aspects, but not in combination; thus any distance margin should not be assumed to cover all cases, and in some may be very small.

The QRH ‘in-flight’ distances represent the manufactures ‘actual’ values and usually differ from those in the AFM; also they have different assumptions about flight path and speed control. Corrections and safety factors should be added to these because again the baseline distances might not be achievable in routine operations.

Interestingly both Airbus and Boeing QRH’s landing data have temp adjustments (e.g. Boeing 737, for max autobrake, approx +-100ft per 10C either side of ISA).
The Airbus table (OLD ? a/c type not known), only adjusts for temps above ISA.

[Basil]

PEI_3721, Thank you for the clarification.
I DID say "Simple pilot answer" and, having looked at your impressive CV, I am sure you are aware that we use approximations in day to day operations, e.g. descent point = HATx3 +7 +/_ wind allowance, timings in visual circuit etc and then adjust as the flight progresses.

I would say that a safety margin is, by definition an 'about' arbitrary figure.
I've a feeling my 'about 60%' is x1.67. E.g. AFAIK structural safety margins in aircraft may be from 1.5 to 3.0 but in ships, up to 6.0.

Re the B737 temp correction; at SL 45C that would add 300ft which is covered by the 60%.

the resulting distances involve many assumptions, predominantly variation in flight path control, speed, and some environmental aspects, but not in combination
Hence my closing comment which event I have witnessed twice; once as a passenger in an Aztec (almost went into a ditch) and once as FO in a Viscount which burst all main tyres. (Yes, I did point out the problem to the PiC but he continued - more work needed on FO assertiveness ;))

[PEI_3721]

Basil, thanks for the reminder of the need for practicality; a distant past.
However, simple rules-of-thumb often make assumptions which if not understood or considered in context can unknowingly result in operations close the limits of safety. Landing performance is one such area, and continues to irritate me; – I’m biased by experiences in aiding investigations into unnecessary overrun accidents, nearly all involving assumptions.

A continuing (practical) problem for the industry is the interpretation of requirements for landing distance; see several related Pprune threads.
The dispatch requirements are clear, relating to the AFM data – no temp correction. What is far from clear is what a safety factor actually covers. A recent ICAO study could not identify a definitive answer, but there was a history of research undertaken by RAE and FAA in the early 60s – the advent of the jet age, which resulted in the current values.
Also lacking clarity is if the dispatch factor applies to a pre-landing assessment; regulations are ambiguous but tend towards ‘not applicable’.

The UK CAA attempted to get to grips with the factor in AIC 14/2006 (now superseded?) – “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.” N.B. ‘remote’ has a specific meaning to regulators in certification.
The AIC continues “Landing distances achieved operationally are affected by the variation in the values of various parameters from those assumed at certification. These include, threshold speed: height over threshold, time from threshold to touchdown (float): delays in initiating stopping procedures; level of braking applied, runway friction.”
This neither includes Temp correction nor excludes it.
What might have greater importance is how representative the factored AFM distances are in actual operation? 30%+ of the factor could be consumed in achieving a normal ‘good’ touchdown relative to the certificated data, and even more in the assumptions about a wet runway, quality of runway surface, tyres, etc. Thus the actual safety margin between the dispatch data for ‘foggy’ wet conditions, or for monsoon wet (but not contaminated) with poor tyres can be significantly different, and in some cases there is no safety margin. Thus any assumption that Temp is within the factor could leave you short of runway.

The AIC concluded – “However, the existence of these allowances should not be taken as an indication that there will always be adequate protection against an overrun if there if excessive threshold speed onto a limiting runway with questionable braking qualities. In arriving at the factor to cover residual variability such as height at the threshold, 'float' before touchdown, delays in commencing braking procedures, substandard wet runway braking action, the effect on landing distances are combined statistically, not arithmetically. It cannot be assumed that the scheduled landing distances can accommodate a landing in which all relevant parameters are at the limit off the tolerance in the adverse sense.” N.B. the value of the limit of tolerance is not specified.

Many investigations and studies have shown that a pre-landing assessment is useful in helping pilots to identify a safety margin for the landing, particularly if conditions have changed after dispatch. Also, that dispatch and pre-landing data are likely based on differing data and assumptions.

JAR/EU OPS requires a pre landing check of distance, but fails to specify what is ‘safe’.
Again the UK CAA provides some guidance in Safety Note 2011/16 “The operator is required to demonstrate that it can ensure safe operations, and by applying the same additional safety margin to the in-flight performance assessment as to the dispatch assessment demonstrates an equivalent level of safety.”
The manufactures have agreed a common format for reporting, and will use new and more practical Operational Landing Distances (OLD) – see UK safety note. In addition, factored OLD normally 15% (FOLD), may provide similar distances to that in the dispatch data.

Thus even the more complex view of the landing issues as above fails to identify conclusive evidence about Temp correction; this appears to be an OPS issue, not Part 25.
If landing distance is marginal and the assumptions not fully understood (too simple), then the absence of a Temp correction might be significant.
A way forward; ask Dassault, compare with Airbus and Boeing.
Will Dassault be using OLD, if so are they Temp corrected?

[renzo.zubi]

Well thank you all for all your comments!

I think that it is strange to calculate the landing field length with precise inputs and complicated garaphics (in Dassault's AFM) and disregard such an important factor like the temperature! It might have a historical reason but it still doesn't make much sense just multiplying this inprecision with a factor of 1.67 or 1.92 to operate safely.

So operators in cold countries are safer than the ones flying in warm countries??

[PEI_3721]

Renzo, “So operators in cold countries are safer than the ones flying in warm countries”, yes if the pre-landing distance assessment is based on the AFM data.
Alternatively, using the manufacturers ‘actual’ distances (QRH) plus corrections and factors, then the risks in landing could be lower, but everything depends on what adjustments and factors are made, an understanding of the basis of the manufacturer’s data, and as always how accurately the approach and landing is flown.

Does Dassault publish ‘actual’ distances in the QRH; do they differ from the AFM?

[renzo.zubi]

Well - the landing data in the QRH do not corrected for temperature. There is only a simple table considering Pressure Altitude and Landing Mass.

The next time I fly the aircraft I will check if the landing performance calculated by the FMS considers the OAT - but I don't think so....

[renzo.zubi]

To add to my previous comment: The landing table in the QRH is only published for a wind-component of ZERO!

[PEI_3721]

Renzo, try AC 91-71 'Runway Overrun Prevention' …
This confirms that Temp is not in the AFM, and then suggests that temp is required before take-off !!
Similar conflictions in operational material re hazards, factors, and corrections, where temp is/is not referred to.
Perhaps a simple adjustment as per wind, but using still air TAS at higher temp vs TAS for a std day.
Back to: – Ask Dassult, cite Airbus, Boeing, and advisory texts on the need to avoid an overrun.