TAS and altitude

[disco87]

I've been reading about why Jets climb as high as possible, I'm quite comfortable with the changes to SFC and also minimum cruise drag on the airframe. However I'm struggling to get my head around why there is an increase in the TAS.

As far as I'm aware, above a certain altitude the mode of speed management is in relation to the MN. Therefore this would result in a decreasing TAS as you climb at a constant MN, however it seems that the TAS increases from what I've been reading, which has confused me a bit.

I'm just looking for an explanation of why this is the case.

Thanks

[bekolblockage]

I'll leave it for the Tech log experts to voice their opinions but from an En-route ATC perspective, I can tell you we do notice a drop off in the radar groundspeed readout (and hence TAS in nil wind gradient conditions) as aircraft pass about F280, transition to MN and continue to climb. Probably only in the order of 10-15 kts or so.
Of course their speed picks up a little again when they level off and accelerate to cruise MN. A/C like the A330 climbs at about M.80 and cruises at about .83, which equals about a 15-18 kt increase.
So, I think your first statement is true assuming all other things are equal from my observation over many years.
Standing-by to be shot down if what I've said is incorrect.

[Da-20 monkey]

If they climb at constant IAS first, and then constant Mach number, the highest TAS will be at the altitude where they change over from IAS to Mach climb.

They climb higher for better fuel flow.

There is a graph that shows Mach, TAS versus altitude, but cannot find it on the net.

[eimin]

In std conditions at SL IAS and TAS are the same. As the airplane flies higher there's less air impacting the pitot tube and thus less total pressure for the same speed.
With less density, IAS decreases. To determine the TAS we have to corrected the IAS for air density, which is a function of altitude and temperature... :)

[John Farley]

At sea level IAS=TAS

At 40000ft IAS is roughly half the TAS.

Drag depends on IAS

TAS gets you places

[Empty Cruise]

TAS only increases up to cross-over altitude, then drops off again. The rest of the climb sorts out SFC. How high you go depends on your optimum level for the winds, actual mass and cost index you operate at.

If you increase the mass, increase the headwind and increase the cost index, optimum level drops considerably....

[FCeng84]

I agree with everything stated in the posts above. Let me add a little more prespective.

The relationship between TAS and Vcas (or IAS) is a function of air density. As altitude increases, air density decreases and thus the TAS for a given Vcas goes up. If your airplane is Vcas limited (in the absense of winds) flying higher will result in covering more ground per hour.

The relationship between TAS and Mach Number is a function of temperature. The colder the air, the slower the speed of sound. For a standard atmosphere profile, Mach 1 at sea level is about 660 knots TAS. At 35K feet, Mach 1 has dropped to about 575 knots TAS. The standard atmosphere profile has temperature constant above 35K feet (up beyond altitudes for all commercial aircraft). If your airplane is Mach Number limited (again in the absense of winds) ground speed will decrease as altitude increases until reaching the iso-thermal rejoin starting at about 35K feet.

The fastest ground speed for an airplane's normal operating envelope is usually found at the Vmo/Mmo corner. For lower altitudes, the max velocity is Vcas limited. For higher altitudes, the max velocity is Mach Number limited. Depending on the aircraft, the altitude of the Vmo/Mmo corner will vary. Most commercial aircraft have a Vmo/Mmo altitude of between 25K and 31K feet.

[pattern_is_full]

However I'm struggling to get my head around why there is an increase in the TAS.

Flip the question around. If you climbed at a constant TAS, your indicated airspeed (IAS) would be decreasing as you climb. Because the air is getting thinner and fewer molecules are ramming into the pitot tube.

Eventually you'll stall, or at least run out of lift to climb further. Because the wings are dependent on the same airflow as the pitot.

Conversely, climbing at a constant IAS means you have to fly faster (in true speed), in order to shove enough air into the pitot to keep the IAS constant.

At the same time, the thinner air makes it easier to fly faster, since there is less air to push out of the way (drag). Up to a point (air-breathing jets lose power as their air intake drops).

So it is kind of a carrot and stick reason - you NEED more TAS to maintain safe/efficient IAS as you get higher - and there is the obvious benefit of getting there faster (since you can).

As far as I'm aware, above a certain altitude the mode of speed management is in relation to the MN. Therefore this would result in a decreasing TAS as you climb at a constant MN, however it seems that the TAS increases from what I've been reading, which has confused me a bit.

Right - Once you shift to a fixed Mach as your target speed, TAS decreases with altitude. If you happened to have a plane that had no (or a very high) Mach limit, then TAS could increase all the way to cruise altitude. E.G. the Concorde, which flew Mach targets, but constantly increasing ones, flew increasing TAS to well above normal airline cruise altitudes.

[JIC]

Simply because of the MACH formula.

MACH = TAS/ LSS

Tas increases with altitude
LLS ( local speed of sound) decreases with falling temp. And temp usally falls with inceasing altitude.

Cheers.

[MD83FO]

One other reason to climb into cooler air is engine efficiency, where the engines can produce higher thrust as there is a higher internal temp margin. This is benefitial up to the tropopause where the temperature flattens out.

[disco87]

It seems cleared up now, thanks for the input.

[Da-20 monkey]

Also, when constant mach climb is selected at the crossover point the vertical speed will increase, because from there on the aircraft is actually converting speed to height gain.
Not that I ever saw this in real life, they told me in a class room. :rolleyes:

[CATIIIB]

Atmosperic pressure at 43000' is 160mb.

So why is IAS is about half TAS? Surely IAS should be 16% of TAS here?

[john_tullamarine]

You might be looking at the wrong numbers and sums. Density is the go rather than pressure ..

It's pretty hard not to get involved with compressible flight at those sorts of levels but, for a first cut, let's just ignore that and treat EAS to be similar to IAS in value.

TAS = IAS/SQRT(sigma) where sigma is the ratio of the actual to SL density.

See, for instance, here (https://en.wikipedia.org/wiki/True_airspeed).

If you take the sigma value for a level, as can be found here (http://www.pdas.com/e2.html) and do the odd sum, you'll see that root sigma (using John's example at FL400)) comes out pretty close to 0.5, hence TAS is around twice EAS (or IAS if we don't get too fussed about compressible niceties).

At FL430, root sigma is a bit less at 0.463 and TAS will be a bit more than twice IAS.

Because density is tied up with pressure and temperature, you can run the equations using pressure but sigma is far easier.

[Pugilistic Animus]

A slightly off topic but related thread (http://www.pprune.org/flight-testing/489290-machmeter.html)

[CATIIIB]

Thanks.

The sums make sense but the logic doesn't!

[megan]

One other reason to climb into cooler air is engine efficiency, where the engines can produce higher thrust as there is a higher internal temp marginRather than the term "higher thrust", I would put it as greater jet velocity from the exhaust, the lower temperature at altitude provides a relatively greater combustion gas energy. Thrust reduces with altitude, at 35,000 = 39.2% of sea level, 50,000 = 18% for a tubojet. Don't know how the addition of a fan, where it can produce some 75% of the total thrust, would be influenced.

[scifi]

I get it... basically what you are saying is that all Air speed Indicators ( ASIs.) are faulty, as they do NOT measure Air Speed.
So we have to use these inaccurate Instruments, and apply 'Fiddle Factors' until someone creates a true Air-Speed reading Instrument.


I think someone will maybe come up with a frictionless Wind-Mill driven device, in the near future. This could revolutionise air transport, and herald a new era in aviation.
.
Didn't the Wright Flyer have something similar...?

[Capn Bloggs]

The relationship between TAS and Vcas (or IAS) is a function of air density.
Not pressure. The air's half as dense at 400, so

Because the air is getting thinner and fewer molecules are ramming into the pitot tube
an aeroplane flying at the same indicated airspeed ie molecules per second into pitot hole will be going twice as fast over the ground, which is True Air Speed when there's no wind component.

The pressure has dropped, but so has the temperature, which also affects the density.

[DaveReidUK]

I get it... basically what you are saying is that all Air speed Indicators ( ASIs.) are faulty, as they do NOT measure Air Speed.
So we have to use these inaccurate Instruments, and apply 'Fiddle Factors' until someone creates a true Air-Speed reading Instrument.

The term "Air Speed" is meaningless without qualifying whether it's IAS, TAS, etc.

An instrument that displayed TAS wouldn't be particularly useful as few, if any, of the aircraft's flight characteristics are TAS-dependent.

[scifi]

Hi Dave, Unfortunately 'Air-Speed' is an absolute concept, it is the speed at which air molecules pass a certain point.
It is only because we use a Pitot driven device that the terms TAS, IAS, and CAS have come about, to get over the deficiencies of the ASI.


Surely all we need to know is the Air-Speed, and wind velocity to work out our Ground Speed, to tell us how long our journey will take.
.

[DaveReidUK]

Surely all we need to know is the Air-Speed, and wind velocity to work out our Ground Speed, to tell us how long our journey will take.

I'd suggest you try flying at, say, 150 kts TAS at sea level and then attempt the same thing at FL300.

You might change your view about TAS being the only important parameter.

[Derfred]

I've been reading about why Jets climb as high as possible, I'm quite comfortable with the changes to SFC and also minimum cruise drag on the airframe. However I'm struggling to get my head around why there is an increase in the TAS.

As far as I'm aware, above a certain altitude the mode of speed management is in relation to the MN. Therefore this would result in a decreasing TAS as you climb at a constant MN, however it seems that the TAS increases from what I've been reading, which has confused me a bit.

I'm just looking for an explanation of why this is the case.

Thanks

Generally correct except for the bit about TAS increasing after climbing at a constant MN. It doesn't. TAS decreases when climbing at a constant MN, due to the fact that the temperature decreases, and therefore the TAS associated with that MN decreases with temperature. Until you reach the tropopause, above which the temperature ceases to decrease and therefore the TAS remains constant with MN no matter how high you climb.

The tropopause will vary considerably due to location, seasons and weather, but is typically around 35000' in a standard atmosphere.

[john_tullamarine]

Not pressure. The air's half as dense at 400, so

Not quite. It is root sigma which is half the sea level value ... but you have the general idea.

[megan]

An instrument that displayed TAS wouldn't be particularly usefulSome light aircraft (think some Pipers were one) had ASI's which showed TAS. Don't really know of what practical use it may be, other than saving reaching for the E-6B.

http://www.experimentalaircraft.info/images/true-airspeed-indicator.jpg

[Owain Glyndwr]

scifi
Dave Reid is perfectly correct, "Air speed" without an adjective is meaningless.
Technically, a pitot static system measures a quantity known as Equivalent Airspeed which is the airspeed which would be registered at sea level by the number of molecules which pass by per second at the flight altitude and speed. EAS is the quantity you need to calculate the forces on the airframe, or to know how close you are to flight envelope limits. In short, it is the number you need to know to FLY the aeroplane. IAS,CAS are simply corrections to EAS to account for variations arising from the positioning of the pitots and static ports along with compressibility effects.
True airspeed, on the other hand, will differ from EAS by considerable amounts which depend on the altitude and ambient temeperature. It is the number you need to know to NAVIGATE the aeroplane.
I must say I am surprised to find this sort of discussion in a professional pilot's forum!

[wiggy]

...Anyway back to the OP and specifically :

I've been reading about why Jets climb as high as possible,

FWIW ..we certainly don't routinely climb as "high as possible" (i. e. Which in our case would be FMC max)..,,as I recall it years back from CFS days design RPM (on a small turbo jet ) came into the best flight level equation....and suspiciously enough these days (and ignoring winds) our best flight level on the 777 (and I think similar was true on the 744) seems to where the N1 was around 90%.....and usually a thousand feet or more below "as high as possible" ..... in my old age am I misunderstanding best range/design RPM?