Landing distance v/s Performance

[niksmathew24]

Hi all,
Just a question regarding landing distance needed with the performance on the day.
I am CPL holder with no jet experience, so I've been reading up on jet engine performances for quite some time and my doubt is:

Does the landing distance increase or decrease on a low performance day? (ie; with high temp and low pressure)
Please do give an explanation on this as I've heard both the possible answers with contradicting reasons and am not quite happy with the reasoning provided.

Thanks in advance,
Nick

[FE Hoppy]

Higher pressure altitude = longer landing distance and lower climb gradient.

[john_tullamarine]

Thrust effects we can discount largely - the variation at idle for ambients is not going to be a major concern.

Main concern will be weight which will determine approach IAS. Hotter and higher will increase the TAS so you would expect, other things being equal, to see an increase in distance with increasing OAT and Hp. As a first approximation, the distance will vary as approach speed (specifically GS) squared.

If your chart doesn't provide for OAT variation, entering the Hp scale with density height works well as a starting point.

Not significantly different to your smaller piston GA Types.

[niksmathew24]

Thanks for the reply guys. I am considering a sea level aerodrome with high temp and low pressure. So I am looking for a simple answer like,

i) You will have a longer run because there is a lesser amount of air and to amount for the poor performance, you're having a higher power setting and poor performance by the engine while using reverse thrusters.

or

ii) You will have a shorter run because the performance of the engine and aircraft is already low.

[john_tullamarine]

I am considering a sea level aerodrome with high temp and low pressure.

The tapeline height of the aerodrome (sea level in your instance) largely is irrelevant as the aeroplane doesn't have a tape measure. What counts is Hp and OAT (both of which the aeroplane understands) and which define the performance heights (ie Hp and Hd). For takeoff and the like OAT is important in its own right as the jet engine is quite sensitive to OAT - far more than for the piston powerplants.

You will have a longer run because there is a lesser amount of air

No .. going faster is what does the trick. That is related to the higher Hd's giving you a higher TAS ... but "lesser amount of air" should have been left behind after your first pre-solo briefing ...

you're having a higher power setting

at idle ? .. or am I missing something here ?

while using reverse thrusters.

not relevant for scheduled distance.

In the real world, for normal conditions, reverse largely is for the pilots' amusement - brakes are where it all happens - unless you are on a long runway and choose not to use brakes. On the other hand, contaminated/wet runways are a different animal if the brakes are near useless.


You will have a shorter run because the performance of the engine and aircraft is already low.

fanciful nonsense. Landing performance principally is due to the brakes and the interaction of brakes and speed .. the engine doesn't have much to do with it.