Originally Posted by
CVividasku
Because it has literally no use.
Let's try to get some meaning to it.
The simple calculation mass*30+300m with A/BRK LOW and mass*20+300m with A/BRK MED will give you a result that is correct 100% of the time where you don't have outstanding tailwind, and a correct braking action (5 or 6, when the LOW/MED deceleration target can be met), to a precision within 100 meters.
So it's not that you're not doing the perf calculation. It is that it can be done with a sufficient level of precision in your head.
The on board performance tool asks you for temperature, QNH.. What kind of difference will one hPa of QNH create ? Around one meter per hPa. One degree ? Three meters. One knot headwind ? 8 meters (tailwind contrarily is significant). Flaps 3 ? Add 100m.
(these are all ballpark figures based on A320 at medium mass with a dry runway)
Do you really think that a figure with an accuracy to one meter has any meaning ? You are splitting hair.
Do you know immediately by heart the touchdown distance that is assumed by the computation ? Because if you care about 10 meters on the result, you should. Also, you should be able to control your touchdown point to around a tenth of a second.
I don't think I'm able to touch down with a precision anywhere like this.
So yes, I prefer to have a real analysis of the situation, intellectualize it, rather than stuff numbers into a computer and mindlessly read the result. I will happily launch a computation in case of any doubt. 99% of the time, the rule of thumb above will have more precision than the touchdown point.
The touchdown zone is 600 meters long !!
When we landed at particularly long runways, I always computed the perfs with the worst possible failure. All brakes fail. The captain would wonder how I ended up with such a result. Pointing that the runway is long enough without the use of brakes proves that the computation was unnecessary...
Also, this computation shows that the perf tool is pessimistic. For the case of a long runway with no braking, the actual performance with reverse idle is close to the computed performance with reverse max.
My comment was specifically designed as an answer to yours. You said that 700 feet on final was not the time to wonder about performance calculations.
You usually compute the perfs in cruise, at least 30-40 minutes before landing. The ATIS that you use is going to be older than this.
Let's say you computed the perfs with the latest ATIS and put the wind into it.
You have 3 knots less of headwind, and you learn that on short final when cleared to land... what are you going to do ? Go around ?
You receive a new ATIS with two more degrees or 1 point QNH less. What are you gonna do ? Ask for a hold to re-compute ?
So, to sum up, a pilot should have a sufficient mastery of the perfs. Knowing how to do a calculation is not enough.
You also need to know the hypotheses that are used, you should have an idea of the result before calculating it, in order to be able to detect a gross mistake. You also need to know approximately the influence of each parameter on the result.
I would be surprised if the guys who took off with a 100 ton error and struck the tail (happened to several airlines) had this kind of mindset.
Ballpark thinking will allow to detect gross mistakes like so.
I don’t know under which regulations You do fly, but this is what EASA has to say about it (as already anticipated by the excellent post of
Speed_Trim_Fail )
AMC1 CAT.OP.MPA.303 In-flight check of the landing distance at time of arrival — aeroplanes
a) The required landing distance for dry runways, determined in accordance with CAT.POL.A.230(a), contains adequate margin to fulfil the intent of the assessment of the landing distance at time of arrival (LDTA) on a dry runway, as it includes allowance for the additional parameters considered in that calculation.
(b) The required landing distance for wet runways also contains adequate margin to fulfil the intent of the assessment of the LDTA on such runways with specific friction-improving characteristics, as it includes allowance for the additional parameters considered in that calculation.
(c) When at the time of arrival the runway is dry or is a wet runway with specific friction-improving characteristics and the overall conditions, including weather at the aerodrome and runway condition, have been confirmed as not changed significantly compared to those assumed at the time of dispatch, the assessment of the LDTA may be carried out by confirming that the assumptions made at the time of dispatch are still valid.
So, assuming you have done your landing dispatch calculations beforehand and that the above conditions are met, you are “good”. If you haven’t then you are not, even on a 4000m dry runway.
With ref. To the rules of thumb calculations suggested at the beginning of your post, I must say that in 31 years of flying Airbus FBW I have never came across it… and back in the day there was no Flysmart, or Octopus.
Calculating your IFLD gives you a good idea of where you will be vacating and your stop margin, increasing situational awareness, especially in case of floating and landing at the very last bit of the touchdown zone, which is still legal, but eats up a lot of runway.
Flysmart contains protections with respect to head/tailwind considerations and varying conditions. It is all described in the related manual.
Tip: if You are a FO, don’t cut corners, that is likely the attitude that could put you in troubles for a future upgrade.
You need to think out of the box when there is not an apparent solution, not when there is one…