Landing weight estimation

[leon737]

Do any common practices exist to evaluate the corrected FOB for landing (landing performance calculation and running through the briefing)? The EFOB displayed while on cruise I believe is the rough estimation, because of route deviation and possible shortcuts. I assume the naive method to add xxx kgs for that specific airfield when arriving on that specific time of day is little bit not scientific. Any thoughts or ideas or just let it go as is?

[ScepticalOptomist]

Unless your landing weight is critical an estimation is more than adequate for the purposes of landing distance calculations and associated ref speeds.

[leon737]

Absolutely. I refer to the specific conditions like short runway or runway is wet or contaminated. A general rule to just add less than 500kgs of gas to the estimation should probably work in most circumstances, doesn't it? I'm talking about 737 and 320 family and exact figures in calculations don't matter in this discussion.

[ScepticalOptomist]

Absolutely. I refer to the specific conditions like short runway or runway is wet or contaminated. A general rule to just add less than 500kgs of gas to the estimation should probably work in most circumstances, doesn't it? I'm talking about 737 and 320 family and exact figures in calculations don't matter in this discussion.

I’ve always used the estimated fuel remaining as the actual fuel remaining at touchdown will be less. This gives a conservative estimate to the actual gross weight on landing.
Again, this is mostly academic unless you are really performance limited.

[AmarokGTI]

Do any common practices exist to evaluate the corrected FOB for landing (landing performance calculation and running through the briefing)? The EFOB displayed while on cruise I believe is the rough estimation, because of route deviation and possible shortcuts. I assume the naive method to add xxx kgs for that specific airfield when arriving on that specific time of day is little bit not scientific. Any thoughts or ideas or just let it go as is?

a rule of thumb at the place I work is just to take the take off weight and minus only flight fuel to get estimated landing weight. Ie for weight purposes plan that taxi fuel, instrument approach fuel, variable Reserve will not be used.

in reality some of those allowances will be used in full (or in part) so the estimate is conservative. Works fine.

[PEI_3721]

“… mostly academic unless you are really performance limited.”
But isn't the point that you need a good estimate of weight in order to determine if you are performance limited ?

If the estimated weight is in error, so too threshold speed for the landing distance calculations. Speed, is a squared term in energy, thus for ‘stopping’ it could have a significant effect on achieved performance.

[oceancrosser]

This sounds like a solution looking for a problem. ScepticalOptomist has nailed it. If you are so tight that a knot to or from decides whether you can land at the destination, the suitabilty of the destination should probably be given a rethink.

[FullWings]

I would hazard a guess that you probably know the mass of fuel on board to greater precision than the rest of the aeroplane and its cargo. Hold baggage is generally weighed but passengers (and crew) and their effects aren’t.

If an extra tonne is going to take you off the end of the runway, then maybe you should be landing somewhere else? Despatch LW check before aviating should see to that?

Getting the aircraft down in the right place at the right speed is more important. If you take the same aeroplane at slightly different weights but land it at the same speed (which you calculated was OK), how does it affect the stopping distance...?

[pineteam]

The EFB landing computation will factor a margin of 15%. We just used whatever the MCDU landing weight predicts. I don’t see the point of having such an accurate landing weight. If you can dispatch the aircraft surely you can land if the aircraft status stays the same. Also I just ran some numbers: Landing at 60T versus landing at 61T on A320 the difference is less than 15 meters with max braking.

[Check Airman]

Knowing the landing weight to the pound is neither necessary nor practical. "Measuring with a micrometer and cutting with an axe" comes to mind.

[ScepticalOptomist]

“… mostly academic unless you are really performance limited.”
But isn't the point that you need a good estimate of weight in order to determine if you are performance limited ?

If the estimated weight is in error, so too threshold speed for the landing distance calculations. Speed, is a squared term in energy, thus for ‘stopping’ it could have a significant effect on achieved performance.

You already have a good estimate of weight given to you by estimated fuel on arrival plus your ZFW.

You are over thinking the need for that accurate of a weight - on my aircraft type (for a given set of conditions) we require a factored landing distance of 1818m at our maximum landing weight and would approach at 157kt.
At our usual landing weight (which is over 20,000kg less) we require 1700m and would approach at 149kt.

Knowing our exact landing weight isn’t necessary, nor practical. If we were trying to land on a runway that was only 2000m long, a more accurate weight would be beneficial, but not critical.

[swh]

Absolutely. I refer to the specific conditions like short runway or runway is wet or contaminated. A general rule to just add less than 500kgs of gas to the estimation should probably work in most circumstances, doesn't it? I'm talking about 737 and 320 family and exact figures in calculations don't matter in this discussion.

You would only increase landing distance by less than 200 ft per tonne on an A320 with a medium poor runway at sea level. If you cannot land with the weight at top of descent you should probably not be going there, that is just not sensible on a contaminated runway as the 737 has demonstrated time and again.

[vilas]

Small difference in LW is not critical for LDR. In QRH the LDR figures are given for max LW. Normally you couldn't be landing higher than that. Even for good to medium braking any credit for less LW than that is mere 10mtrs for flaps full and 20mtrs for Flap3. If overweight then penalty is 40mtrs per T for Flap full or 3. So it's really splitting hair. Nothing is achieved by being so accurate for a ton or less.

[PEI_3721]

Sceptical “… thinking the need for that accurate of a weight …”

Maybe, but its the thinking in general which raises concern.

If operations are based on approximations, then what about ill-considered ‘approximations’ (uncertainties); e.g. the reported wind speed, runway conditition, all of which affect landing distance. How might these add up to erode the distance safety margins; which combinations are more critical on a length limited runway.

Yet crews manage uncertainty everyday; which suggests that judgement of the approximation is good enough, but when might it not be, when does it becomes the critical factor.

xxxx m runways are only sufficient up to a point; how is the point of sufficiency judged, the point at which approximate landing weight is still good enough … when we are near a limit, … but that requires more accurate assessment of where the limit is, i.e. less ‘approximation’ … etc …

‘… common practices exist to evaluate the corrected FOB for landing …’; whatever value / rule of thumb is chosen always consider the range of outcomes; safety margin in LDR vs LDA. This is increasingly important on short wet runways where margins decrease, thence approximations are more critical. #12, swf :ok:

[FullWings]

I think the main point is if you land your jet at a particular speed because that correlates with the weight you *think* you are, as long as you are not grossly wrong in your estimations, the landing run will be pretty much the same, whatever weight you *actually* are. Much in the same way that if you tried to land even a lightweight aircraft at Vref+80 (has been tried), you’re probably going off the end unless it’s a very long runway.

That said, errors have a nasty habit of compounding, so you should use the most accurate available data in calculation but be aware of the tolerance in the result. Accuracy vs. precision and all that.

[PEI_3721]

FW ‘… right place at the right speed is more important …’ :ok:

vilas, you words “small”, “critical”, #13, could mask the importance of the approximation by falsely assuming sufficient accuracy.
When does ‘small’, ‘critical’ matter. If the approximation is critical, then to know this requires knowledge of the difference between not-critical and important.

I assume that your “… LDR figures are given for max LW” relates to the max LW in conditions at the time of assessment; configuration, runway braking action, choice of auto-brake setting, etc.

“Nothing is achieved by being so accurate for a ton or less”, but this could be the vital factor in combination with other approximations or mistaken values; e.g. normal range of error in wind speed, braking action vs wet or very wet, type of runway surface, depth of tyre tread, plus the variable human.

[vilas]

FW ‘… right place at the right speed is more important …’ :ok:

vilas, you words “small”, “critical”, #13, could mask the importance of the approximation by falsely assuming sufficient accuracy.
When does ‘small’, ‘critical’ matter. If the approximation is critical, then to know this requires knowledge of the difference between not-critical and important.

I assume that your “… LDR figures are given for max LW” relates to the max LW in conditions at the time of assessment; configuration, runway braking action, choice of auto-brake setting, etc.

“Nothing is achieved by being so accurate for a ton or less”, but this could be the vital factor in combination with other approximations or mistaken values; e.g. normal range of error in wind speed, braking action vs wet or very wet, type of runway surface, depth of tyre tread, plus the variable human.
My comments are on difference of few tons of weight. Don't you use factored landing distance? That will take care of that difference. One ton extra increases Vapp by 1kt. Do you fly that accurately? If you don't have twenty or thirty meters extra you should not be landing there.