Quick question in thin air

Hey community, I have a question: Does landing at high elevation airports require a higher speed because the air is thinner?

I have done this recently and the attitude of the aircraft suggest this, indicator speed for the a321 seems to work well around 165-170

Thank you so much for your input!

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It doesn’t require a higher IAS (correct me if I’m wrong), but does require a higher GS, hence the usual longer runways.

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Thank you, for some reason, the aircraft had pre-stall behavior and it does this with me at high elevated landings, I cannot figure this out.

And ground speed is always much higher than the indicated airspeed which is really confusing me.

(With weight at minimum)

Ground speed should be way higher than IAS at cruise. If you’re talking about takeoff/landing at sea level then that’s weird.

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Yup, it’s at landing and you are correct, with me, it is always that the gs are much higher than indicated which is normal, but it still pitches up as if it is about to stall until I take manual control. It is weird and it happens often.

Maybe somebody who has authority can look at my stats during my most recent flights.

Make an attempt to read the post carefully, he is asking about higher altitude airports.

In short, yes you have a slightly higher landing speed but nothing too extreme I don’t think.

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Your airspeed at landing at high altitudes will be the same as at sea level airports. But when you’re at high altitudes the air is thinner as you know, so 140kts IAS at a high altitude will have a higher groundspeed to it, hence the longer runways.

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You have all answered this, thank you so much, team! This can be closed.

Before it’s closed:

Less dense air makes the wings a little ineffective. The engines find it harder to run as they get less air for combustion, meaning the pilot has to run with more throttle, but the resultant thrust will be less.

The runways are made longer to make the landings and take-offs easier. The engines won’t give out enough thrust with less dense air, so the plane needs more time to accelerate to take-off speed, hence longer runway required.

While landing, the longer runway allows higher approach speeds, allowing the pilot to remain in a comfortable approach speed. The air flowing on the wings is also not very dense, increasing the risk of a stall in lower speeds.

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The pitot static system relies upon the air entering the pitot system creating the dynamic and static pressures required to ascertain your airspeed (along with temperature of course). As you climb the physical density of the air entering the end of the pitot tube reduces thus reducing the dynamic pressure for a set given speed. Hence to maintain a ‘set’ IAS the ground speed will increase as you have to force more ‘thinner’ air into the pitot tube to maintain a ‘set’ dynamic value.

When landing at higher altitude airports (e.g. NBO), especially when hot, it is the effective Density Altitude you should be looking at. NBO for example is about 5000’ AMSL often with temperatures in excess of 30 degrees. Under ISA standard atmosphere regs the temperature at 5000’ AMSL ‘should’ be 15 degrees - 2 degrees per 1000’, so 15-10=5 degrees. 30 degrees will give you ISA +25 and an effective ‘density’ altitude of approximately 7500’.

This will give you higher indicated airspeeds (the AOA of the wing would have to be slightly higher to counter the thinner atmosphere to give the same required lift vector necessitating a slightly higher IAS and more thrust to maintain a normal pitch approach) and associated higher ground speeds. The flare will be less effective (less dense air to ‘grab’) and the threshold speeds will be higher. The go-around profile will be affected and the engines, for heavy take-off weights, might be thrust limited due to temperature.

Most high bypass gas turbines are ‘tuned’ for high cruise altitudes with a fixed front spool and optimised IGV’s to the core so the density altitude differences at low altitude (below 10,000’) will have little effect. Turbo props and piston engines are slightly different and are slightly more affected but the differences are so minimal as to not be a factor. Aerodynamics and speed will rule or ruin your day.

HTH.

Edited to iron out ambiguities!!!

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Wow, I never knew that.

I think that’s why when I tried taking off at mt Everest airport my airspeed in the 757 was 160 before I lifted. We were 10,000 msl!

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I fly with a higher speed because of that, I loose control if I’m below 140 IAS at KDEN, I usually fly it at IAS150.

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Thank you @Yuan_Tugo another well written an educational post.

To help put it into context please are you able To give an example of landing speed at NBO as compared to back at LHR for a similar aircraft such as a a B777?

Many thanks and happy landings

David

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It’s a little more tricky than that. The 777 has Vref based on weight and we are more interested in the landing distance calculation than the speed. The aircraft will happily cope with a small Vref variation due to temperature and altitude.

However, from the landing distance calculation tables you can see the effect of energy on the aircraft derived from height above sea level and temperature!

So, for example, to calculate the landing distance we start with a reference distance for a given weight of 250T (777-300ER) and a set autobrake setting.

e,g, For 250T and Autobrake 3 a reference distance would be approx 2500m.

We then calculate the variables which include:

Weight adjusts for each 5T above and below 250T
Altitude adjusts per 1000’ above MSL
Wind adjusts for each 10kts head or tail wind
Slope adjusts per 1 degree slope + or -
Temperature adjusts for each 10 degrees above or below ISA
Vref adjusts per 5kts above Vref 30
finally:
Reverse thrust adjusts.

As you can see above the altitude and temperature factor in here as the energy is taken into account for high altitude airfields. So, for NBO for example, the airfield is 5000’ above AMSL and, from the graph, I have to add 70m per 1000’ so 5 x 70 = 350m for the altitude.

The temperature is, in this case, also +70m per 10 degrees over ISA. So 30 degrees at 5000’ is ISA + 25 (ISA temp ‘should’ be 5 degrees at 5000’) giving 70 x 2.5 = 175m for the temperature.

Therefore, not taking into account any other variables, just for the height and the temperature the corrected landing distance required for the 777-300ER for NBO at 250T is 2500m + 350m + 175m = 3025m required. An ‘extra’ 525m just for the extra energy carried over the threshold.

HTH.

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Denver is a very high city. Ive been there and it’s pretty thin air!

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