IAS to Mach Number?

[Mamakim]

Hi All,

Why do big jets when climbing past (for eg) FL200 to FL300, change from flying an IAS to a mach number? Is this for ATC seperation reasons of some sort or something else?

Thanks people...

MK

[john_tullamarine]

From the point of limitations,

(a) at lower levels, EAS (IAS for practical chaps) is limiting to constrain dynamic pressures on critical areas of the aircraft,

(b) at higher levels mach is limiting to constrain compressibility problems.

For a practical climb, the aircraft initially flies a constant IAS until the level at which the desired mach number is achieved .. thereafter, the climb is at constant mach. The schedule normally is somewhat within the AFM maximum speeds.

Descent is the reverse.

[pressed_on]

I havn't flown jets, but have read the theory and done the Turbine course etc so correct me if I'm wrong :uhoh:

This is really just elaborating on the above post but is it to do with controlling the 'relationship' between A/C speed and the Local Speed of Sound(LSS)? which is essentially what flying at a constant mach no does. As you climb higher LSS decreases etc so it may be easier to set the Mach no instead of winding down the IAS knob all the way up to FL340 etc? Crusing at a mach no would also mean that any changes in air temp, density, etc along the route would be corrected for by the A/C instead of the Pilot getting out the abacus and spilling the coffee everywhere;)

Standing by for corrective bitchslap...:ouch:

[tinpis]

Gawd wots the mystery?:ugh:
Ya climb the bloody thing at whatever the books tell ya to ....300kts into M.70 or whatever
Theres a bloody big dial/speed tape in fronta you that tells you all this stuff.
Theres nothing HARD about it.:hmm:

[neilia]

Just to elaborate a little further... as the aircraft's speed approaches the speed of sound, all sorts of undesirable aerodynamic effects begin to kick in. Hence, the speed is restricted in relation to the speed of sound, which is why Mach number is used - Mach number is the ratio of your speed to the local speed of sound, ie 0.7 Mach means you're doing 0.7 times the speed of sound.

[Chimbu chuckles]

The LSS reduces as you climb higher...if you climb at a constant IAS you will soon enough exceed Mmo as it is a fixed % of LSS...so around 29000' the aircraft, automatically, changes it's speed reference to hold a constant Mach number...holding a constant mach number leads to reducing IAS which also helps with climb rate...climb high enough and the twin effects of reducing LSS and reducing IAS to remain at or below Mmo leads you to 'coffin corner' where you can no longer accelerate nor decelerate without experiencing high or low speed buffet.

Having said that most modern aircraft have to be significantly in excess of Mmo before buffet is felt.

We routinely cruise the 767 right on the barber pole and as a result occasionally experience transient overspeeds and the aircraft behaves impeccably with the only difference being a warning beeper. I think the limit is more to do with increased drag/fuel flow penalty to cruise faster than any nasty mach effects.

[RENURPP]

occasionally experience transient overspeeds
CC, out of interest does that require an inspection?

[Chimbu chuckles]

Nope. We operate to CAA certification limits so our MMo is about .02 less than FAA limits...a transient overspeed does not even require a written report to the boss. Even exceeding the FAA limit is a non event in practical terms.

Edited to put the 0 and the 2 in the right order.:ugh:

[RENURPP]

Even exceeding the FAA limit is a non event in practical terms.
Is in ours, the bloody thing tells on us.:*

[Chimbu chuckles]

Our Boeings don't have the 'big brother transmitter':ok:

[john_tullamarine]

CC, out of interest does that require an inspection?

For Types certificated to Vmo/Mmo, the limit is a line operations animal .. and can be exceeded intentionally for authorised test flights and pilot training .. refer FAR 25.1505 (http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=00bdb6d5f77b8eea713c43a1c8af2af1&rgn=div8&view=text&node=14:1.0.1.3.11.7.198.3&idno=14).

As any who fly such beasties can attest, it is entirely usual to see the needle wander a little above and below the barber pole during a descent with any turbulence.

[404 Titan]

Start doing that sort of thing in any FBW Airbus and the bl**dy thing will pitch the nose up to protect itself. Best thing to do in the FBW Airbuses is keep away from Vmo/Mmo.

[Chimbu chuckles]

Indeed...years ago I was required to carry out post C check maintenance test flights on a Falcon Corporate jet.

One part was exceeding Mmo by some set amount I cannot remember now...Mmo was .865 and it took two goes to achieve the required test point...I was too gentle the first time so had to climb back up to F410 and try again...after rolling over to get a steeper dive angle, and leaving the thrst set at MCT, it still took a fair push to overcome mach trim..having achieved about .9 or a little above I then was required to release the controls and extend the speed brakes and record the angle of bank we ended up with and the angle of bank remaining after stowing them...then they got rerigged and we go up and do it again.

Was very interesting...and fun...there was a fair bit of rerigging to do as we ended up at around 45 degree AoB which reduced to about 20 when I stowed the brakes passing through about F330 in a very steep dive.

Falcons are so wonderfully designed aerodynamically and overbuilt strength wise I had no fears of things falling off...I am reliably informed the Dassault TPs exceed M1.0 in them.:D

Edit; Yes 404..it amazes me the French designed both my all time fav aeroplane and the one I hope to avoid for the rest of my career:E :ok:

[404 Titan]

Chimbu chuckles

Don’t quote me on it but I think the B777 does the same thing though I think if you push hard enough you can still override it. I’m sure some Tripler driver though will clarify what I have said as fact or not.