RAS/TAS/mach in climb & descent

[mystic_av8r]

Can someone please explain in simple language the procedure used on the big jets? i.e selection of climb speed, changeover to mach (when and why?) and descent speed selection??

[alatriste]

The procedure is simple: let´s figure out that the recomended climb speed is 315/0.74.
Initially you will mantain 315 KIAS, as you are climbing Mach number will increase until certain FL when 315 KIAS will be 0.74 M, this will be the moment to switch to Mach number. Mantaining this constant M your KIAS will decrease as you climb.
The moment your KIAS equals your desired M number depends on OAT, it should be about certain FL (270-280) but it depends on ISA deviation.

For descend you should the reversal procedure: Mantain de initial M number until you reach desired KIAS.

I hope this little explanation will help you. REGARDS.

[john_tullamarine]

Changeover is not related to OAT but is a function of pressure only. For a given schedule the changeover will occur at the same Hp regardless of OAT.

[alatriste]

I would like to obtain a further explanation by the moderator.

YOU ARE RIGHT, I have checked my AFM and there is a chart where you can convert Indicated Airspeed into True Mach number depending only on FL.

Anyhow I don´t understand the reason . For a certain FL (constant PA) and certain EAS ( IAS corrected by instrument and compressibility), I think ISA deviation will change density altitude and therefor TAS for the same EAS.

If OAT is higher there will be a higher TAS at same FL and EAS ( depends on relative density), when OAT increases speed of sound also increases ( depends on relative temperature). As far as decreasing ratio of temperature and density are not equal, How the relationship of airspeed to the speed of sound can remain constant?

Thanks again for your time.

[Genghis the Engineer]

Playing around with equations for a moment:-

We all know the following...

M = TAS / SQRT(Gamma * R * T)

and EAS = EAS * SQRT (Sigma)


Thus M = EAS * SQRT (sigma / (gamma * R * T))


clearly gamma and R are constants, so we can take those out, giving...

M = k * EAS * (sigma / T)^½


So, accepting that as you climb you start at a EAS value (we'll assume it's the same as IAS which is a reasonably approximation) and switch to a Mach number, the changeover point comes when...


M(climb)=k*EAS(climb) * (sigma / T)^½


Since M(climb), EAS(climb) and k won't change, we get a changeover condition of...


(sigma/T)^½ = M(climb) / (k*CAS(climb))



Just making a quick check on this, at FL200 (the height selected arbritrarily) my ISA tables show for ISA the left hand of this equation is 0.0463. Using the ISA-15°C table however it comes out at 0.0493.

So it would appear that the changeover condition isn't always at the same altitude.

Having said that, last time I was asked to calculate a climb profile we simply assumed ISA and since no Vne / Mne limits were approached I don't think anybody troubled greatly about the innacuracies and small reductions in optimal climb performance - we did declare a fixed changeover flight level.

G

[john_tullamarine]

I shall have to dig out some books and play with the equations .. .... back in due course ....

[Keith.Williams.]

Mystic,

Before this string goes too much further, it might be helpful if you were to comment on whether or not you are happy with its current trajectory.

I note that you asked "Can someone please explain IN SIMPLE LANGUAGE the procedure used on the big jets? i.e selection of climb speed, changeover to mach (when and why?) and descent speed selection?? "

I'm note entirely convinced that the string is meeting your requirements (I'm pretty sure it is not, but could be wrong).
Although much of the material in the previous posts is probably accurate, a large proportion of the ATPL students with which I deal would not thank me for explaining things in this way.

[mystic_av8r]

Thank you for all replies posted so far.

All I need to know really is a step by step description of speed scheduling (which speeds are used, RAS/mach changeover, at what point and why...) thru from initial climb to TOC, and from TOD to start of the approach. The first explanation from Alatriste seems to make sense, just angling for some explanations from a different perspective. Come on you airline guys, just LIST what you do!

[timzsta]

This topic has come up a number of times on Tech Log. I wrote a reply (the day after Isat my ATPL Principle of Flight exam) on an earlier thread. You can search for it using "Mach TAS" .
Speeds/heights are based around a 737-300. No arguments please about speed of sound versus temperature please tefal heads - it is the way it is in aerodynamics.

Lets say take from Stansted, RWY 23, flap 5, acceleration altitude 1150 AMSL (800ft AAL).
Today lets say VR is 135 kts and V2 (take off safety speed is 150kts).
So at VR pull the stick back up to pitch attitude commanded by Flight Director.
Positive rate of climb - gear comes up. LNAV and then VNAV engaged to follow SID.
Accelerate to V2+20kts (170).
At acceleration altitude, nose is lowered to allow acceleration to clean speed. Around here vertical mode of autopilot can be changed to N1 or MCP SPD/VS. Dial up say 230kts and 2000fpm climb.
Flaps will be up between 210-220 kts dependant on aircraft weight. Accelerate to 250 kts.
Handover to London Control - "no speed maintain XXXXft on reaching, squawk ident etc"
Dial up 300 knots in the speed window and suitable ROC, best not to use LVL CHG or you go up a tad quick and set everyones TCAS off. So 300kts and 2000 fpm is a sensible starter.
We have to stay in MCP SPD and not VNAV as the FMC is programmed to command climb at 250 kts below FL100. On passing FL100 engage VNAV and the FMC will climb the aircraft at about 310-315 kts.

Afraid a little theory needed here:

As we climb the air temperature is decreasing so local speed of sound is decreasing. We are climbing at constant IAS and as air pressure and temperature are decreasing our TAS is increasing. So we have a problem. If we continue to climb at constant IAS we will eventually have a TAS = speed of sound. Well not quite but it will reach what is called Vcrit - the point where at some point over the aerofoils or the aircraft the velocity of the air passing reaches the speed of sound. In reality we have speeds called Vmo and Mmo which represent the max IAS and Mach respectively we can fly at without encroaching on Vcrit. Exceeding Vcrit will cause buffet and shock stall over the control surfaces - which is not good for us. Remember the air accelerates as it passes over the wing - so you can get supersonic flow over a conventional airliner wing at about Mach 0.75.
To solve this problem (exceeding Vmo in a climb at constant IAS and exceeding Mmo in a constant mach descent_ we change from climbing at constant IAS to constant Mach at around FL260 and vice versa in a descent.

So FL260 - FMC commands climb at constant mach (about 0.74) vice constant IAS (310kts).
TOC - FMC commands cruise at cruise speed, typically about 0.74 again.
TOD - FMC commands descent at constant Mach about 0.74, thrust levers are retarded and speed is controlled largerly by pitch.
FL260 - cease descent at constant mach. Typical FMC profile will be 300kts descent. This is all the reverse of what was explained above.
FL120 - begin reducing to speed 250 kts for speed restriction.
FL100 - 250 kts
Downwind - reduce to about 220 kts
Base - reduce to 180 kts, flap extension starts at about 210 kts.
Once established localizer most airport ATC want 160 kts til 4 miles from a 737 size aircraft.
4 miles from touchdown, reduce to approach speed Vref (which must be 1.3 V stall).
Over the treshold - Vref
Touchdown - Vstall x 1.15
Reverse, spoilers, 80 knots, manual brakes, "next exit ground 1xx.xx" etc.

Hope thats what u wanted.

[mystic_av8r]

Thankyou very much Timzsta......thats exactly what I wanted.