Haroon one of the ‘five or more opinions’ on how to judge the landing distance required is to consider the factors in the certification and assumptions in preparing the normal, factored landing distance – knowledge and then application.
The 1.67 factor (1.92 if wet) provides a margin of safety to reduce the probability of an overrun by allowing for variability in normal landings.
The airspeed and altitude accuracy expected at the threshold are critical factors. In an emergency, a pilot might claim to fly with greater accuracy, and achieve the expected touchdown position. However, assuming that you are not well experienced in overweight landings, and with the stress of the unusual situation, then some allowance should be made for possible unfamiliarity with the aircraft at that weight. Possibly a slower pitch/power response or operating nearer the wing lift curve limits – wings bend more at high weight.
These points would suggest considering the touchdown at approx 1200ft beyond the threshold as a minimum (probably 500ft above the unfactored distance) and a further 500ft for not deploying spoilers / reverse, and not braking as quickly as the test landings demonstrated in obtaining the data. Then add another 500ft for possible +10ft, +5kts at the threshold (AC91-71), or should this be +10kts – does everyone land fast at night. Do not unduly pressure yourself, stay as close as you can to normal operations.
You did not specify if the unfactored distance was wet or dry, or included the airborne distance (Boeing data usually doesn’t include the 1000ft+ from threshold to touchdown) – know before you go.
So including air distance, you need 7200ft (5700+500+500+500), but without air distance at least 8200ft – check wet/dry. Then ask yourself why the dry (1.67) factored distance is 9500ft, and how many overruns have there been this year.
Consider the brakes; any factor less than 1.67 probably requires max brakes until you are absolutely sure of stopping; is there likely to be a brake energy limit. Also consider the runway surface, wet/dry, grooved or smooth, and how much overrun is there. Check runway slope and wind values if not in already the book factor – double check the wind, make allowance for any reporting errors.
You may not wish to, or have the capability for a go around (obstacles), thus plan a more conservative approach and provide additional margins.
After considering all of the above, consider again what the landing distance factor is for – a margin of safety; what further addition would
you add for safety.
You state that there is an option to burn off fuel. Safety must always come first; take option (B), as any non-normal (overweight) landing will involve some risk. Even with a much longer runway why take the risk on commercial grounds –
you still have to answer for the consequences if anything goes wrong.
If you have to land immediately then accept reduced margins of safety – greater risk, but always consider the operational consequences; consider what mitigating actions can you take. By doing this, you pre-prepare your answers; lawyers usually state “always do what is right – do what you can justify”.
Beware advice from other people, they may not understand the situation from your point of view, or the safety of the operation; don’t assume that others have the same understanding. Yes, consider all advice (use all resources), but
you have to decide on the safest course of action.
Fuel cost and time do not always equate to safety, the pilots first priority is safety. If you think that an overweight landing has the same level of safety as a normal landing then be prepared to prove it.
Re the ‘factors only apply at dispatch’; many regulatory airworthiness authorities imply that the factor should still be used at destination after checking the actual conditions. Remember that in the event of an accident, the regulators may have to defend their position; if a factor is required at dispatch to reduce the probability of an overrun, then what has changed at the destination – are they going to accept a higher probability of an overrun. Whichever way the regulators call the game, the pilot holds the final accountability in the decision to land.
Landing performance of large transport aeroplanes.
AC 91-71 Runway Overrun Prevention.
Managing Threats and Errors during Approach and Landing.