From Mach to airspeed in descend and climb 737

[Olendirk]

Guys,

when does the mcp panel switch from speed to mach and vice versa?


when does the FMS switch from speed to mach and vice versa?


thanls so much


OD

[Pennellino]

Official changeover flight level for speed/mach is FL260

[paperdragon]

Hmm,
depending on AP mode beiing used...V/S or LVL CNG, it won´t, at all! Though max min speed reversion will force the speed window to change numbers if you hit em! But VNAV yes FL 260!

[ppppilot]

I think it is not in a fix point neither an altitude. In your fms you will see the speeds your AP will maintain during climb and descent. During climb it will maintain IAS. The Mach number increases with the altitude maintaining the same IAS. When required mach number is reached then it will maintain that Mach Number and the MCP changes to Mach. On the descent works opposite. It will maintain the Mach until it matches with the required IAS. So the point of change depends on the speeds calculated by the FMS and the point both matches which is related to TAT (True not total).
Imagine it was a fixed level. Depending on the outside temperature you could exceed the Max IAS or Mach Number.
Nice flights.

[Denti]

The classic (well, dunno about the 400, but 500 and 300 work that way) speed control switches form IAS to Mach at FL260 if in level change or V/S.

In VNAV climb it will keep the IAS in the mode selected (ECON most likely) until the corresponding mach number of that profile is reached and then keeps that. That way switching usually is done later, often around the FL280 to FL310 mark depending on cost index and entered top of climb wind.

During VNAV descent its just the other way round, the FMC keeps the mach number according to the selected cost index and switches to IAS when that is reached. Descent speed schedule is not variable but rather fixed speeds according to the selected CI. With a low CI it often happens that the switch to IAS is done immediately upon selecting VNAV descent, for example we use a fixed CI of 20 which corresponds to 266 kias descent speed and that is often enough reached at our normal cruising levels, since 266kias isn't really ATC friendy most of us change the descent speed on the descent page to 280kias which is reached a tad later.

[Sky Pilot]

I don't think it's a fixed point. But more to do with VMO and MMO.

During a VNAV climb the FMC will climb at an IAS / Mach No. dependent on cost index, expected cruise level, predicted winds and mode eg ECON. MAX ANGLE or MAX rate. As you climb at a constant IAS Mach increases and when IAS equals the desired Mach No. the FMC automatically switches to a constant Mach climb. eg. if you are climbing at 280kts and the Mach No. is 0.79 the FMC will switch at the altitude where 280kts equals M0.79. As Mach is temperature dependent this will occur mostly at a specific temperature not a specific FL.

If you are climbing in Level Change or V/S then the logic is slightly different. As you climb you are initially IAS limited. eg 340kts. At sea level this equates to about 0.55 Mach. As you climb 340kts eventually equals MMO eg 0.82 Mach and as you climb further 340kts indicated will exceed MMO. Remember in the cruise we often fly at 0.79 with an IAS of 240kts or less! At the point where VMO equals MMO the aircraft becomes Mach limited. You can see this when the barbers pole or black and red bar begins to reduce. This is the point at which the MCP will change over to Mach. Again it is mostly temperature related. It usually occurs at around FL260 but is not dependent on the FL. Usually the aircraft will switch from say 280kts to M0.69 or so! To continue to climb at this will mean the IAS will begin to decrease and so it is prudent to change back to IAS until you reach your desired Mach No. and then change to Mach. If you don't the aircraft will eventually overspeed when the IAS equals MMO.

In the descent the opposite is true but I'll let you figure that out for yourselves!!

[john_tullamarine]

As Mach is temperature dependent ..

Don't think so ...

Pt/P = (1 + (g-1)/2 * M^2)^(g/g-1)

where g = 1.4, typically.

.. speed and FL dependent. Crossover height will depend on the speed/mach combination in question.

[kme]

Interesting point John can you please expand or post a reference for us with a little less theoretical background.

Thanks
Emil

[ppppilot]

I can not understand those complicated formulas cause my mind has room enough only for a couple of “precision approach to girls” procedures. Using simple words:
Mach number is a function of true airspeed and air temperature whereas IAS is largely a function of true airspeed and air density. Consequently, when an aircraft descends at a constant IAS, the Mach number reduces but when an aircraft descends at a constant Mach number, the IAS increases…
From AAIB mouth
http://www.aaib.gov.uk/cms_resources/dft_avsafety_pdf_033318.pdf (http://www.aaib.gov.uk/cms_resources/dft_avsafety_pdf_033318.pdf)

[john_tullamarine]

Mach Number can be determined solely from measurements of dynamic (ie Pt-P ... say, CAS) and static pressure (P .. say, pressure height) .. temperature doesn't come into it at all.

There's probably not much point in quoting this and that form of equations as they tend to confuse those who aren't used to playing with them .. however, most of the standard engineering texts will provide endless arithmetic confusion for those who might wish to flagellate themselves mercilessly ....

This thread (http://www.pprune.org/forums/showthread.php?t=53558) has a few more thoughts ...

[Beg Tibs]

In a climb in VNAV (IAS window closed) it will self transition from IAS to MACH as entered on the FMC climb page

In a climb in LEVEL CHANGE it will transition at around FL260 and it will then climb at M0.69 in the speed window

In a descent in VNAV it will fly the programmed MACH until it reaches the IAS as entered on the FMC descent page. If the IAS is lower than current cruise MACH (slowed down for ATC or using CI20 or less usually) it will reduce thrust and enter a slight descent aiming to be at IDLE and IAS at the same time

In a descent in LEVEL CHANGE it will maintain that speed regardless as it makes its way to the red bricks...

[latetonite]

Answer to John : I learned : a=20VT(k) ..no temp in the formula??

[john_tullamarine]

Answer to John : I learned : a=20VT(k) ..no temp in the formula??

sonic velocity certainly depends on temperature ... but we're talking about Mach Number ... which doesn't.

[Stick Flying]

I think you will find the local speed of sound is accounted for in the CAS calculation. It may be so that you can determine these from pressure calculations but pressure calculations also include temperature elements. So the answer is yes, Mach has a speed of sound element (for subsonic flight, as most of the replies retlated to).

Just as most textbooks describe (A.C Kermode- Mechanics of Flight).

[BOAC]

Keeping it 'simple' for ppppilot.

MCP flies you at IAS. Let's say 280kts?

2 things are happening as you climb

1) At 280kts IAS, your TAS is increasing
2) Local Speed of sound ('TAS') is reducing

Thus, a 280kt IAS climb in your 737 to 41000 feet will have you flying at around Mach 1.0 when you get there, starting at around M0.4 at sea level. As a rough guide, the S of S at 41000 is roughly 580kts ('TAS'). I don't know what 280kts IAS at 41000' is, but I guesss.....around 580kts....?

As said above, 'official' change over is quoted as 26000' but that depends on air mass and selected speed.

[blackmail]

hello every one,

there is an easy formula that says it all. but first an assumption : forget the "position error" between ias(indicated airspeed) & eas(equivalent airspeed resulting from pitot pressure). so ias = +/- eas & assume atmosphere as roughly std.

eas = 661 x M x Vdelta.( i could elaborate how to find this formula but kiss, keep it simple st...d). 661kts = speed of sound at sea level. M = Mach n°, greek delta = ratio p/p0 ; p0 = 1013hpa. now the crossover altitude/flight level we are looking for depends on which climb schedule we want to fly or which one we programm in the fms. e.g :280kias/M.76. so 280= 661 x .76 x Vdelta . delta is the only unknown factor. so first at lower level we climb at constant eas, delta(pressure ratio) decreases with altitude so Mach(much lower than .76) must increase to keep eas cte. at a certain altitude/fl M= .76 & eas = 280: we are at the crossover alt/fl . we calculate delta & look in a std atmosphere table(which we all carry in our kitbag, don't we?) for the corresponding fl.
in our example delta = p/p0= 0.31 = fl290.
i totally agree, in normal ops, you let the automatics do this math.above the crossover alt/fl we climb now at M cte, delta still decreases with altitude so, now ias/eas decreases. in desent the whole process is reversed.
an analog formula is availabla for true airspeed: TAS =661xMxVtheta. theta=T/T0. T0=273+15=288Kelvin at sea level std.
i need a cup of coffee now.
bm.

[abenk1984]

where to find? in FCOM or FCTM?

[enicalyth]

John T is asking I think that you perform a little substitution in two thermodynamics equations.

The first equation should be familiar, that of temperature rise with Mach Number? You use it to calculate total air temperature,yes? Page 171 of Hill and Peterson on Mechanics and Thermodynamics of Propulsion is one reference.

T2/T1 = 1 +[(g-1)/2]*M^2 where g is the ratio of specific heats also called "gamma".

The second equation is on the same page and deals with pressure rise

P2/P1 = 1+ e[(T2/T1)-1)]^[g/(g-1)]

In this equation e is the adiabatic efficiency which we'll say is unity.

Re-arrange the first equation to obtain

[(T2/T1)-1] = [(g-1)/2]*M^2

Substitute that into equation two and you have

P2/P1 = 1 + {[(g-1)/2]*M^2}^(g/(g-1))

So provided that the process is adiabatic and 100% efficient if we know the pressure ratio P2/P1 and assume that the value of gamma is pretty much constant at 1.4 then the Mach Number pops out.

Some authors use a slightly different presentation [Saravannamuttoo, Rogers] but I'll stick to JT's version.

And he did say Mach Number not the speed of sound.

Hope this helps and I hope I have not mistyped.

The "E"

[homerj]

I was just about to say that enicalyth

[chksix]

An excellent page for aerodynamic data:
Aerospaceweb.org | Atmospheric Properties Calculator (http://www.aerospaceweb.org/design/scripts/atmosphere/)