IAS/MN cross over height

[Cost Index]

I was taught this a few years ago but i'm having trouble rembering why this technique is required. Could someone elaborate and explain with reference to what happens to IAS, EAS, TAS and MN as we climb using this technique?

From memory the altitude where this cross over occurs is where a jet will also fly at it's fastest GS, true? Or is was it something else that occurs at this height other than the above?

Cheers :D

P.S. Graphs would be great as they help me understand :E

[Keith.Williams.]

To get a general idea of how EAS, IAS (or CAS), TAS and Mach vary with altitude in the ISA below the tropopause, just hold your right hand out in front of your face so that you see the palm of your hand, and your fingers point upwards. Now spread out your fingers.

Your little fnger represents EAS, the one next to it IAS (or CAS), the next one is TAS and the next is Mach. The palm of you hand represents low altitude and your fingertips represent high altitude. The left edge of your palm is low speed values and the right edge is high speed values.

To see what happens in a constant IAS (or CAS) climb rotate your hand so that the IAS (or CAS) finger points straight upwards. The directions of the other fingers show how the other speeds vary. You little finger is sloping left indicating that EAS is decreasing. The other two fingers are sloping right indicating that TAS and Mach are increasing. So in a constant IAS climb the EAS decreases and the TAS and mach number both inrease.

Now rotate your hand so that the Mach finger points straight upwards. All of the other fingers are sloping left indicating decreasing EAS, IAS and TAS. So in a constant mach climb in the ISA below the tropopause, all of the other speeds decrease.

Maximum allowable speed at low altitude is limited by VMO which is a CAS. Maximum allowable speed at high altitude is limited by MMO which is a mach number.

To avoid exceeding either of these limits jet aircraft climb schedules are specified in terms of CAS and Mach number. For example 270 Kts / M0.8. This means that the climb is initially at a constant 270 Kts CAS.

As altitude increases at this constant CAS, the mach number gradually increases. The crossover altitude in this case is the altitude at which the 270 kts CAS is equal to Mach 0.8. When this altitude is reached the climb switches to one of constant mach number at Mach 0.8.

During the constant CAS part of the climb the TAS is increasing. During the constant Mach part the TAS is decreasing. So the greatest TAS occurs at the crossover. Assuming still air or constant wind speeds at all altitudes, this means that greatest groundspeed in the climb occurs at the crossover altitude.

[betaboy]

Is it correct to say that for the Y-axis, you could replace Altitude with Density, hence solving problems involving temperature changes at constant altitude?

[Old Smokey]

betaboy,

No, that's not a good idea. The CAS / Mach Changeover altitude is entirely dependant upon Pressure (i.e. Pressure Height) in isolation from Temperature and Density. The maximum achievable TAS for the given Mach / CAS schedule occurs at changeover height. Wind variations can give you you highest GS at virtually any height.

The TAS arising from a particular Mach Number is entirely dependant upon Temperature, in isolation from Pressure and Density.

The only time that Density (as a Density Height) comes into the speed formulae, is in the conversion from EAS to TAS. The older methods of using CAS and Density Height to find TAS actually yields DAS, Density Air Speed. DAS is acceptable with minor error for low and slow operations, but inappropriate for higher and/or faster operations, where eith the 'F' factor is applied to DAS, or, more appropriately, conversion to Mach Number is made from CAS and Pressure Height, and then, using Temperature, find the TAS.

Regards,

Old Smokey

[timzsta]

Chicken Tikka Massala (CTM) is the way to remember this.

Climb : - CAS TAS Mach +
______________________

Descent : - Mach TAS CAS +


Eg climbing at constant CAS, TAS & Mach Number increase.
Climb at constant Mach, TAS and CAS are decreasing.
Descend at constant Mach, TAS and CAS increase.
Descend at constant CAS, mach number and TAS decreasing.

The four examples above highlight nicely what is happening as you jet climbs or descends through the changeover height.

(written by fATPL 200hr wannabee who stands ready to be corrected - was a while ago I did Performance:ok: )