VMO to MMO

[Vone Rotate]

Hi all,

I going through the books trying to find the answer to a question and am struggling.:ugh:

At what altitude does the Mach meter come alive and you fly Mach number instead of IAS and you monitor MMO rather than VMO?

Whats is it that causes the cross over? I remember from my MCC when the Mach meter springs to life you fly that and ignore the ASI but I cant for the life of me recall the factors involved. Altitude? OAT? air density? All of them? I seem to recall 26000ft for some reason....

Thanks in advance and apologies if this has been covered before but I couldn't find it by searching:ok:

[Checkboard]

The Mach to IAS conversion is fixed for each pressure altitude (ie Flight Level), thus the Mach number is said to be independent of temperature. The operator specifies a climb & descent speed schedule, so one operator might say "Climb at 280 knots IAS into Mach .76", while another might say "Climb at 300knots into Mach .78" - so the Flight Level at which you change is determined by the operator's SOPs.

In terms of when Mach number is displayed (IAS is always displayed) - this depends on the instrument type & fit.

[Vone Rotate]

That makes sense.

Just to clarify, say the SOP is climb at 280kts into M0.76. For aguments sake this cross over happens at FL280.

Is this the same for each flight/ weather conditions?

As an example no matter the weather/ OAT/ air density 280kts and M0.76 will always meet at FL280?

Don't mean to pick the bones out of it but I have an interview coming up and a friend of mine got asked about this last year in an interview.
I'd kick myself if I go asked it and had never bothered to find the answer...

Thanks again:ok:

[Type1106]

Quite simply - the actual level at which the changeover occurs is dependent upon the ambient temperature and pressure. You cannot say it always occurs at FL280 - it'll be around there but will vary on the conditions.

Just fly the thing at the scheduled climb IAS until the Mach meter reaches the scheduled climb Mach - - usually it will be the same as the cruise Mach No. At least it was in my old jet!

Rgds

1106

[john_tullamarine]

Temperature isn't involved. The parameters of concern are the IAS/MN combination.

The following link (http://www.pprune.org/tech-log/314513-mach-airspeed-descend-climb-737-a.html) takes you to a couple of threads with more than enough potential to confuse all but the diehard arithmeticians .....

[easyme]

I fly the Boeing 737NG and therefore can only comment on this aircraft type.

It depends on which mode you are using for your climb/descent;

1. Using LVL CHG the aircraft will fly IAS up to FL260 and the speed will changeover to Mach number and continue to fly the mach number you hae on changeover.

2. Using VNAV it will fly the IAS until it reaches the Mach number assigned in the FMC, this is not dependant on altitude.

[zonnair]

Climbing with e.g. 280/.78 it will maintain 280 until .78 is reached. Then it continues with .78. Unless the speed is manually selected of course.

[Vone Rotate]

Check this explination out! Top marks sir....

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.

That will do. I just wanted to give it a bit of an explination if asked about it.

I fully appricate in the real world there is no need to go this deep each time we climb through the flight levels...

Thanks all:ok:

[Checkboard]

i totally agree, in normal ops, you let the automatics do this math.

Actually, in normal ops (and before the days of EFIS screens, computers and "modern" pilots who neither know nor care about the theory of high altitude flight :hmm: :8 ) we let the circular slide rule do the math.

http://www.pilotshop.com.au/shop/images/productImages/CR-4.jpg

This is a CR-4, I have always (and still do) carried a CR-2. This one is set for a Mach of 0.45, with the CAS/PA window (the longest window) displaying the Airspeed/Pressure altitude relationship for this Mach Number.

[Slasher]

I still carry my CR4 Checkers. Still pull it out on rare
occasion - more so the odd conversion and the wind side.

Sometimes I'll show the kids what the IAS and TAS would
be if the A320 was suddenley slungshot to 80,000ft ISA at .80

[DFC]

At what altitude does the Mach meter come alive and you fly Mach number instead of IAS and you monitor MMO rather than VMO?

Whats is it that causes the cross over?


The altitude at which the mach meter comes alive depends on the mach meter.

The altitude at which mach becomes a reference in a climb is the altitude at which that mach is equal to the chosen IAS.

For example, if you climb at Vmo at low level the mach number will be low, however, as one climbs maintaining a constant airspeed the mach number will increase (even if you can't see it on the mach meter yet) until you find that at Vmo you are also at Mmo.

If you continue to climb at Vmo then you will exceed Mmo so from here up you use mach as the reference to avoid exceeding Mmo.

If you climb at a lower speed, you will find that there will usually be a certain mach that is maintained once that mach is reached. Again climbing at a constant IAS causes the mach to increase until the chosen Mach is reached and thereafter this mach is maintained.

What is it that causes the change over?

The answer is that climbing at a constant ias causes the mach number to increase and eventually the chosen ias will be more than the desired mach number.

Similarly when descending if one descends at a constant mach number the ias will gradually increase until (if you are not watching it) Vmo will be exceeded.

Therefore while mach will initially be the reference in the descent, eventually the desired ias for descent will be reached and from then on this IAS will be maintained.

In simple terms, it is easier to fly at a constant IAS or a constant Mach and the crossover is simply where constant IAS changes to constant mach (or the opposite).

[Old Smokey]

Vone Rotate,

You've quoted a CAS/Mach profile of 280 Kt / M0.76

For this speed schedule crossover will occur at Pressure Height of 31189.1 ft, where the Equivalent Airspeed (EAS/Ve) will be 266.7 Kt, and the Static Pressure 284.96 hPa / 8.415"Hg.

Temperature will be..............Who cares? Temperature is not a factor.
Density Height will be............Who cares? Density Height is not a factor.

For a 280 Kt / M0.76 speed schedule, crossover height will be the same EVERY time, regardless of temperature. Different speed schedules have different crossover heights, for example a speed schedule of 280 Kt / M0.78 will always occur at 32473 ft.

TEMPERATURE AND DENSITY IS NOT A FACTOR!

Regards,

Old Smokey

[411A]

I still carry my CR4 Checkers. Still pull it out on rare
occasion - more so the odd conversion and the wind side.

Likewise...and you would be surprised at some of the younger folks whom have never seen one...in practical use.:}

[Checkboard]

more so the odd conversion and the wind side.
I use it every day for fuel litre/kilo conversions, well not "every" day, but you get the drift. Everyone else I see uses electronic (usually phones) calculators - and I have seen the typical "100 times error" turn up which doesn't usually happen with the whizz wheel (as you have to do a rough sum in your head for that bit every time anyway).

Flying in Europe, I occasionally play with it in cruise on the days I am not reading the paper, to remind me of the days (in Australia) when I was really 250 miles away from a decent airport. At those times, I check the IAS and Mach against the TAS on the FMS - but just from curiosity about the accuracy of the CR-2...

[john_tullamarine]

One point of possible confusion just occurred to me and it doesn't appear to have been visited explicitly in the preceding posts.

If folk are looking at the barberpole, then the limiting crossover will occur at the same level for a given Type as Vmo and Mmo will be the constant TC limits for the Type at the crossover level.

That is to say, the barber pole will follow Vmo on the intial climb - note that, for some Types, Vmo may have a few kinks as the climb progresses - and, when the level causes the Mmo to equal Vmo, crossover occurs. Same each flight.

However, most operators don't fly the climb at barberpole. Therefore, the operator's IAS/IMN schedule will determine crossover quite independent of whatever the barberpole may be doing.

[Slasher]

Note the barberpole crossover level. Its this level that gives
you the highest TAS which is useful if your running real late.
FL260 in the 734 I recall, FL250 in the 320 (selected).

[Canuckbirdstrike]

For the VMO to MMO crossover altitude just look in the limitations section of the aircraft FCOM.

[Slasher]

Flying in Europe, I occasionally play with it in cruise on the days I am not reading the paper....

So do I Checkers especialy on long trips.

And I play with the CR4 sometimes too! http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/icons/laugh.gif

[syedo]

If folk are looking at the barberpole, then the limiting crossover will occur at the same level for a given Type as Vmo and Mmo will be the constant TC limits for the Type at the crossover level.

so JT, if we set MMO on the emer desc, when we reach crossover alt, it will maintain VMO? i am not quite sure on this, but will have a look at it on my next box visit...:O

[bio161]

PLEASE help me guys! :ugh:

I can not get it. Static pressure does not change with temperature as density does?
If i have an increase in temperature i will (at least i think) have for sure a decrease in density and pressure because the air becomes less dense!..So why the MACH is not connected to temperature and density??

The formula of the MACH it self includes the temperature parameter (MACH = TAS / LSS which includes the temperature expressed in kelvin). So if i fly at a given level with a relative lower temperature than usual i should get a given MACH number earlier...or am i missing (for sure..) something? :{

My personal guessing has always been that the change over to MACH from a given IAS is not at a fixed level but changes continuosly according to the actual temperature of the atmosphere. So if it´s colder than usual it will happen at a lower level and if it´s higher than usual it will come at a higher level. But i have the feeling, after reading these posts, i´m saying a lot of nonsense .. :eek:

Thanks mates for your help! I´m really crashing my head against the wall to find the explanation! :ugh:
bio161

[rudderrudderrat]

Hi bio161,

Since TAS depends on a density correction applied to IAS, and LSS varies with temperature (which affects Density Altitude), the net result is that temperature is cancelled out.

i.e. Even if the temperature is different from ISA, for a given IAS at a given FL (But your Density Altitude will be different) = the same Mach Number.

ASI measures (Pitot dynamic + static pressure) - (Pitot static pressure) = ( Pt - Ps)

Machmeter measures (Pt-Ps) / Ps.

[bio161]

Hi rudderrudderrat,

Thanks a lot for your explanation. Short but i have to say precise and clear! There's no better way to do it! :ok:

I just have one more doubt in my mind. I understand that a variation in Temperature will affect both TAS and LSS so, at the end, the result MACH will not vary because both factors vary at the same time of the same (i hope i don't say something wrong now..) amount and they cancel themselves out by their own. So temperature is not a factor. This i got..

Now. Taking back for a moment the temperature of a given air mass a change of this, as you said as well, will affect density altitude.

But now my head crashes again. A change in temperature which is followed by a change in density does not change as well the pressure of the air? :ugh:

So here now i come back to my problems. How can it be possible that we ALWAYS find the SAME pressure at the SAME FL which allows us to make the crossover from CAS to MACH if temperature's changes affect as well pressure? :confused:

Thank you again guys. I'm sorry if maybe i ask something that should be obvious. :\

Regards to all of you,
bio161

[rudderrudderrat]

Hi bio61,

A change in temperature which is followed by a change in density does not change as well the pressure of the air? Sea Level pressure depends on the mass of air over each square unit of area (sq. inch). If you change the temperature, you will have changed the height of the column of air.

How can it be possible that we ALWAYS find the SAME pressure at the SAME FL That's all an Altimeter does. It may be graduated in feet - but it only measures pressure (Pressure Altitude) - you're real Altitude will change with air density.

[bio161]

Hi again rudderrudderrat and thnx again for your reply!

Slowly i'm getting it..The atmosphere does NOT behave in a fixed volume way but this one varies according to the temperature so to keep a constant pressure.

As you said the Altimeter mesaures PRESSURE altitude. That's why we set the QNH every time we fly in order to know our true altitude (without counting now temperature variations of more than 10°).

So correct me if i'm wrong. But may be i got it now:

The changeover between EAS and MACH will always come at the same PRESSURE altitude, which indeed we can find at different REAL altitudes which depend on density, temperature ecc ecc.

Did i get it? :\

Sorry if i keep on asking but i'm starving to get to the solution! :{

Thank you guys again!
bio161

[bio161]

Anyone that could please reply to my question above? :sad:

Thanx guys! :ok:

[Checkboard]

The changeover between EAS and MACH will always come at the same PRESSURE altitude, which indeed we can find at different REAL altitudes which depend on density, temperature ecc ecc.
Yes - exactly correct.

[Old Smokey]

As Checkboard says "Yes - exactly correct":ok: CONGRATULATIONS!

Note - Not butting in to the last post, but because I had a PM along the same lines.

Regards,

Old Smokey

[Rivet gun]

I'm a bit late on this thread. I'm glad BIO 161 has got this sorted. He's not the first and won't be the last to have found this confusing.

I think this is an issue which the flight schools don't teach well. They teach the relationship between TAS and mach number (which depends on temperature) but they dont teach the analogous relationship between EAS and mach number (which depends on static pressure). The equations are simple (one was quoted earlier in this thread)

TAS = M x 661.48 x sqrt(T/T0)
; T0 = 288.15 kelvins

EAS = M x 661.48 x sqrt(p/p0)
; p0 = 1013.25 hpa

661.48 is speed of sound at ISA sea level.

Perhaps a reason for this is that our speed tapes display CAS not EAS. The equation relating CAS and Mach number is too complex for the ATPL courses so they don't teach it. This does not alter the fact CAS and Mach number are related by static pressure. (temperature is irrelevant)

CAS exists only because it is the best you can do with a mechanical ASI. With EFIS, CAS should eventually become obsolete. I think the Eclipse buisness jet displays EAS on the speed tape and so did the space shuttle. Perhaps Boeing and Airbus will eventually ditch CAS. Then compressibility error will be just a historical curiosity.

[bio161]

Thank you all guys! Finally this is clear into my mind as well! :ok: Overall thanx to Old Smokey for the explanation via PM! It has been really helpfull!!

I really have to say that this chapter is not so well explained in the flight schools..But better late than never..isn't it? :}

I just have one further question concerning EAS and CAS. Is there a kind of rule of thumb which tells us the amount of change during our increase in speed?

For example at 200 kts CAS we get 10%less on the EAS due to the compressibility error..or something like that (i'm just inventing) :\

Thank you again!
Regards,
bio161

[Rivet gun]

I don't know if there's a simple rule of thumb for EAS - CAS because it depends on altitude as well as speed.

Remember that at sea level pressure altitude compressibility error is zero even at high subsonic speeds (because that's the altitude the ASI is calibrated for).

Unlike the Mach meter, the ASI does not know it's altitude, so it can only be calibrated free from compressibility error at one particular altitude (which is sea level by convention)

An EFIS does know it's altitude so could theoretically display EAS at all altitudes. In practice most civilian EFIS display CAS in order to be compatable with mechanical ASIs, i.e they deliberatly emulate the mechanical ASI's compressibility error.

If you search the internet you'll probably find a compressibility error correction chart. It is the same for all aircraft.

[bio161]

Great. Now i can say i have this chapter clear in my mind! It's incredible how many things you get to know from one you don't know..the main thing is keeping on willing to learn without being convinced to know everything! :)

Thank you Rivet gun. You have been really kind in answering again! :ok:

Always happy landings to you all guys,
bio161