Landing distance question

[catpinsan]

Landing Distance: "what" is being compared to "what" ?

@Speedwinner,

At the expense of repeating some of what the other excellent posts have said, here's another detailed look. Also, your understanding is largely correct, but here's a structured way to look at, and analyse the issue:

Landing distance calculations come into the picture at 2 stages - the Planning stage, and the Enroute/Destination/Diversion Landing stage.

1. Planning stage: the FLL, Field Length Llimit, landing weight, is derived by taking the destination LDA and finding the weight at which the 'actual' maximum performance Landing Distance factored 60% ( i.e. divide by 0.6 or multiply by 1.67, then termed as the LDR, Ldg Dist Required) results in LDR = LDA.

For a short RW at destination, this could in turn affect the RTOW, Regulated Take Off Weight at the departure point if TOW minus predicted Fuel Burn > FLL Landing Wt.

2. Inflight stages:

After take-off,
(i) if all goes as planned, we have an ample margin already in place as described in 1 above. At the planned destination, this would take care of issues such as variations in piloting technique - "so we can afford a little flare right?", use of brakes - variations in weather to a reasonable extent, aircraft snags resulting in small increases in RW length requirements etc. this is why LDR is not generally worked out before landing, except perhaps to help with 'minimum RW occupancy' criteria to facilitate exiting at a particular RW exit.
(ii) unanticipated conditions, in case of diversion Enroute, landing weight and/or aircraft snag issues could arise, or even at destination, an increased LDR could arise due to tailwinds, aircraft system failure etc. and/or reduced LDA for any reason such as RW closure/change etc. in this case we must work out the LDR.

Having stated that, a few comments on the issues you raise:

A. The easiest first - a Threshold is a threshold, whether displaced or not it stands for the same thing, treat it with equal respect. You have to be 50ft, nominal, crossing the threshold. The Landing Distance from your tables will be from 50ft down as you have already mentioned so this must be compared to the "distance beyond threshold" whatever way that is depicted, in your chart.

B. A simplistic look at margins on 'actual' Scheduled Landing Distace - once airborne, variations are catered for by the regulations by requiring 15% to be added to the actual maximum performance distances demonstrated by the manufacturer.

One manufacturer assumes 50ft over the threshold followed by a flare (technique also described in their documentation) resulting in touchdown between 6 to 8 seconds later. Depending on aircraft type, it is quite normal, to specify a touchdown point range for larger and longer-body jets - 1000 to 2000ft, or for eg for the B747 it started at 1500ft in the age of the 3 bar vasi.

Geometrically a 3 deg angle corresponds to a 5.2% gradient which corresponds to 961ft horizontally forwards for every 50ft of height change, so that's where a 'no-flare' would land us.

Note that 15% of a 6000ft RW is 900ft, so a prolonged flare, for instance, could eat away a substantial part of the TD Point variation mentioned by the manufacturer.

C. PIC discretion can and must always be invoked when the variable factors cannot be enumerated with any degree of confidence - if the assumptions made by taking data directly from Landing Distance charts do not satisfy the real world conditions, certainly add a further arbitrary but satisfactory margin but remember, you must be able to defend such position taken by an argument that shows sound professional judgement was used.

Hope that helped.