Hello simmers,

 

I have been thinking this for a while now and I was not able to work it out. It might be blindingly obvious but I am not sure.

 

My question is what altitude should a jet fly at to give the highest true airspeed (TAS). Because I have noticed that the higher I fly I actually fly slower but I am saving fuel.

 

I think this will be useful for when I am planning a flight that I want to complete a bit quicker than usual.

 

Thank you for your help.

I am no expert in this matter at all, but as far as I am aware, there is no such altitude. It will always be a compromise that depends on the aircraft, its weight and its climb performance.

Also, and that is an important thing to keep in mind, the more significant speed is perhaps ground speed (GS), not TAS. Ground speed will in fact increase with increasing altitude, and that means that the higher you are the faster you can reach your destination, so you're actually looking for the highest altitude your aircraft can reach.

I'm not a real pilot, just a sim pilot, but I do agree that ground speed is of greater importance than TAS.  Aircraft specifications from their manufacturer will often include altitudes at which fastest and most economical cruises can be obtained.  This is important information, but you must also consider the weather.  Your aircraft may fly the fastest at 30,000 ft MSL, but if you must fly into a strong headwind at 30,000 ft and there is far less of a headwind at 20,000 ft, then your fastest cruise on that trip may be at 20,000 ft.

My question is what altitude should a jet fly at to give the highest true airspeed (TAS). Because I have noticed that the higher I fly I actually fly slower but I am saving fuel.

 

At the same power setting, jets are not flying slower at higher altitudes.  The reverse is true.  Jets, especially, NEVER cruise at low altitude simply because they're inefficient down low.  You may be confusing INDICATED airspeed with True airspeed.  They're quite different.  Due to the fact that at high altitude, the air is thinner, meaning, fewer molecules of air into the pitot tube, indicated airspeed drops markedly.  That's why jet speeds often are expressed in percent mach.  

 

As others note, ground speed, which is the combination of TAS and the effect of wind, is what gets you there.  But in terms of aircraft performance, TAS is the important one to know.  Indicated airspeed is less important.

I don't think I explained myself correctly, but I found out that the fastest altitude to cruise at when not accounting for winds is called the "crossover altitude". This is the altitude that the plane switches from IAS to Mach. So if the plane climbs above the crossover altitude then the plane will be flying slower because as altitude increases the speed of sound.

 

 

Here is a link to what I mean. Enter the aircraft's climb speeds, the IAS and the Mach and it will give you the crossover altitude, this is the theoretical fastest altitude to cruise at. Again, the wind is not accounted for no favourable winds at higher altitudes may be faster than flying through headwinds at the crossover altitude.

 

 

http://www.hochwarth.com/misc/AviationCalculator.html#CrossoverAltitude

 

 

I would like to know the formula that is used to calculate the crossover altitude but I haven't done much research about it yet.

What you call crossover altitude has nothing to do with the aircraft's actual speed.

It is merely the point in the upper atmosphere that one switches from IAS to mach simply because mach becomes more accurate and IAS less accurate. You are confused between ground speed, indicated airspeed, true airspeed and Mach.

Your how long to get you there speed is your groundspeed. Assuming the same wind at FL240 and FL420 you will fly faster (higher ground speed) the higher you fly. Why? Because, say you are climbing at IAS 290kts at about FL345 290kts will equal MACH 0.84. You level off at that altitude and your engines will have to be throttled back significantly to stay at M 0.84. Getting to that altitude though is a different ball game as it depends on the a/c performance, thrust and weight.

Your dispatcher will try to strike a balance between economical use of fuel and scheduled time of arrival.

 

Take the a/c top left. That is actually capable of flying well over M0.93 because it's a high performance jet with super critical wings. That's why it is still even today the holder of the fastest commercial Atlantic crossing excepting Concorde!!

Well put it this way, a plane flying at Mach 1 at 5000ft is flying faster than a plane flying at Mach 1.0 at 30000ft.

 

 

the speed of sound decreases with increasing altitude up to 11 km

 

So therefore, flying above the crossover altitude is in fact slower than flying at the crossover altitude. 

Well put it this way, a plane flying at Mach 1 at 5000ft is flying faster than a plane flying at Mach 1.0 at 30000ft.

 

 

So therefore, flying above the crossover altitude is in fact slower than flying at the crossover altitude. 

So therefore according to your theory Concorde would have crossed the Atlantic more quickly at sea level than at FL600 !!

 

 

So therefore according to your theory Concorde would have crossed the Atlantic more quickly at sea level than at FL600 !!

 

Yes that is correct, at sea level (I calculated using 100ft), the true airspeed at Mach 1 is 661 knots, and the true airspeed for Mach 1 at 60000ft is 573 knots. However this would use so much fuel that it would not justify the time savings.

I believe it is because sound travels slower at low temperatures. As the speed of sound, calculated by 331.3+0.606t, "t" by temperature. so it means flying at 30°C at 0.84 Mach is faster than at -50°C at 0.84 Mach