Landing speed
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Landing speed
Just a simple question. Does landing at 5,000ft above sea level require a faster speed than landing at 100ft above sea level? Thanks in advance.
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It depends on what you mean by speed as there are several definitions!
We fly the aircraft based on what we see on the 'indicated' airspeed instrument - this measures speed by sensing how much air pressure (due to forward motion) is entering the instrument and converting it to an airspeed.
The higher you go (e.g. 5000 feet versus 100 feet as you quote), the less air there is and the lower the atmospheric pressure. This essentially means you need to travel quicker through the air to get the same reading on the above instrument. So to get the same indicated airspeed (say 140 knots) at 100 feet takes less ACTUAL forward speed through the air (termed 'true airspeed) than getting 140 knots indicated airspeed at 5000 feet (where the air is thinner).
So, for a given indicated airspeed (which the aircraft is always flown on as this determines lift), the true airspeed is higher at higher altitudes i.e. your dial will read 140 knots (if that's the aircraft's landing speed that day) at 100 feet and 5000 feet but, at 5000 feet, you'll actually be travelling quicker through the air.
All of the above is complicated by another definition - 'groundspeed', the speed you're travelling relative to the ground. This is wind dependent.
We landed yesterday with about 5 knots of tailwind - we usually land with a headwind, which is usually at least 5 knots, say. 5 knots of headwind to 5 knots of tailwind represents a 10 kts difference in ground speed and let me me tell you - you notice it and the landing feels a loooot quicker!
If you look up the definitions of indicated, true and ground speed, I think it will all become a bit clearer.
A simplified answer but hopefully a starting point?
B&S
We fly the aircraft based on what we see on the 'indicated' airspeed instrument - this measures speed by sensing how much air pressure (due to forward motion) is entering the instrument and converting it to an airspeed.
The higher you go (e.g. 5000 feet versus 100 feet as you quote), the less air there is and the lower the atmospheric pressure. This essentially means you need to travel quicker through the air to get the same reading on the above instrument. So to get the same indicated airspeed (say 140 knots) at 100 feet takes less ACTUAL forward speed through the air (termed 'true airspeed) than getting 140 knots indicated airspeed at 5000 feet (where the air is thinner).
So, for a given indicated airspeed (which the aircraft is always flown on as this determines lift), the true airspeed is higher at higher altitudes i.e. your dial will read 140 knots (if that's the aircraft's landing speed that day) at 100 feet and 5000 feet but, at 5000 feet, you'll actually be travelling quicker through the air.
All of the above is complicated by another definition - 'groundspeed', the speed you're travelling relative to the ground. This is wind dependent.
We landed yesterday with about 5 knots of tailwind - we usually land with a headwind, which is usually at least 5 knots, say. 5 knots of headwind to 5 knots of tailwind represents a 10 kts difference in ground speed and let me me tell you - you notice it and the landing feels a loooot quicker!
If you look up the definitions of indicated, true and ground speed, I think it will all become a bit clearer.
A simplified answer but hopefully a starting point?
B&S
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Rob,
B&S answers the basic question you ask - but how much difference between 100' and 5000'? A rough guide is to add approximately 3 knots per 1000' altitude to get TAS
For example: a sea level runway threshold speed of 140kts Indicated Air Speed (which is what the pilot sees on his instruments) results in a groundspeed of 140 kts with no head or tail wind. However for the same IAS, at 5000' the groundspeed becomes approximately155kts. As this speed is obviously the one that affects stopping distance it results in a significant difference in landing distance required.
As I say, a rough guide, because temperature also come into it, but close enough for government work!
1106
B&S answers the basic question you ask - but how much difference between 100' and 5000'? A rough guide is to add approximately 3 knots per 1000' altitude to get TAS
For example: a sea level runway threshold speed of 140kts Indicated Air Speed (which is what the pilot sees on his instruments) results in a groundspeed of 140 kts with no head or tail wind. However for the same IAS, at 5000' the groundspeed becomes approximately155kts. As this speed is obviously the one that affects stopping distance it results in a significant difference in landing distance required.
As I say, a rough guide, because temperature also come into it, but close enough for government work!
1106
Last edited by Type1106; 13th Jul 2010 at 14:35. Reason: To explain that the IAS for the weight remains the same groundspeed changes
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Just a simple question. Does landing at 5,000ft above sea level require a faster speed than landing at 100ft above sea level? Thanks in advance.
But I wonder if you wanted a simple answer to a simple quesiton, so at the risk of over-simplification, here's one. At 5000 ft the aircraft speed over the ground at touchdown will be faster than at sea-level because the air is thinner.
I have to add, though; "other things being equal apart from air temperature and pressure which will be lower at 5000 ft".
(Other things include....weight, wind, pilot's inclination to fly by the book, etc)
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Thanks for three great answers, it is what makes PPrune such a good read.
I had been wanting to ask this question all week after landing at Guatemala City airport.
I was certain we were going faster than “normal” but couldn’t be sure. I was slightly worried but not unduly, after all, if you don’t trust the person flying the plane you shouldn’t be on it in the first place.
I had been wanting to ask this question all week after landing at Guatemala City airport.
I was certain we were going faster than “normal” but couldn’t be sure. I was slightly worried but not unduly, after all, if you don’t trust the person flying the plane you shouldn’t be on it in the first place.
