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.
Last edited by CVividasku; 4th August 2024 at 23:17.