ROC at the crossover altitude

[elcol]

This is an old ATPL (A) question which has raised its head again (why I don't know-its pretty pointless) so I'm looking for an input from a current flier.
The question asks what happens to the ROC at the crossover altitude when transiting from constant IAS to constant mach climb?
Now I believe that there is a momentary pitch up at the transition giving an instantaneous increase in ROC which almost immediately decreases to a value slightly less than the ROC at the constant IAS.
Can some kind aviator please verify this and let me have some numbers and aircraft type.

Much appreciated
elcol.

[Alt Crz Green]

The theoretical answer required by the ATPL may well be different to the reality, so here goes.
Depending on the weight of the aircraft, the ROD may increase slightly as the IAS starts to reduce. It's so slight though as not be to readily noticeable. Working against it is that the ROD is always trending downwards anyway as you climb into thinner air. Any increase in ROD may also be down to the autopilot chasing the speed as it finds it harder to hold it accurately in the thin air. This phenomenon will in fact mask any increase due to mach conversion. Add in varying temperatures will alter the Mach number, thus further causing the autopilot to chase the speed when in the mach regime.
This is for A320. Typical climb is 300kt to conversion (about FL290 depending on temperature) then M0.78. The IAS will then decrease to about 260kt in the cruise.

[Rishy]

ATPL theory answer (I hope):

RoC is contstant with a constant IAS (and increasing MN). At and above the crossover atltitude MN stays constant and RoC falls as IAS falls.

Rishy

[Alt Crz Green]

Rishy, I have no idea what the jaa theory is, but your explanation is not reality. RoC is not constant with a constant IAS in the climb, RoC will fall as you hold the speed due to the thinner atmosphere. On conversion, RoC will continue to fall, but again it's due to the thin air and not because the IAS is falling.
Go back to basics: stick back, aircraft climbs, speed decreases.

[Lord Spandex Masher]

Don't Roger The Moose.

[Rishy]

I'm led to believe that much of the current ATPL theory is quite removed from reality.

[Alex Whittingham]

The JAA exam question is this, with answer (A) marked as correct.You climb with a climb speed schedule 300/.78. What do you expect in the crossover altitude 29 200 ft (OAT = ISA) ?

(A) The rate of climb increases since the constant IAS-climb is replaced by the constant Mach-climb.
(B) The rate of climb decreases since climb performance at a constant Mach number is grossly reduced as compared to constant IAS.
(C) During the acceleration to the Mach number .78 the rate of climb is approximately zero.
(D) No noticeable effect since the true airspeed at 300 kt IAS and .78 Mach are the same (at ISA temperature TAS=460 kt)

I agree with Alt Crz Green

[Artisan]

TAS = Mach x LSS

As you climb, LSS decreases with decreasing temperature.
If Mach is constant then TAS will decrease with decreasing temperature.
Lift is proportional to TAS.
If TAS decreaes, Lift decreases.
Therefore ROC decreases until reaching a (theoretical) constant temperature (-56.5C) at the tropopause.

In Alex's exam question above, I would choose answer (D) as the most correct. At the actual crossover altitude, there is no change in ROC, because there is no change in TAS. Above the crossover the ROC will start to slowly decrease. This is what I observe daily in the real aircraft.

[Rishy]

Sorry folks, my mistake :=

[Georgeablelovehowindia]

Elcol, you had it correct in the first place, and answer 'A' is the one they're after.

Think about it: climbing towards 29,200ft, climb thrust set, 300 IAS, ROC 1000fpm (say), Mach no. increasing ... 0.76 ... 0.77 ... bingo! 0.78. If you maintain that pitch attitude and thrust, the Mach no. will increase, so to stop it doing that, you pitch up, and the ROC increases. For a while, anyway, until as you climb higher, it starts to decrease again.

They didn't ask anything about the TAS, did they? As always, RTFQ and KISS.

Artisan, I don't know what 'real aircraft' you fly, but I'd stick to sailing your yacht chum. My 'real aircraft' were the Boeing 707/720B, Boeing 737-200/300, Boeing 757, and MD DC-10-30, and that's what I observed to happen on all of them.

:ok:

[elcol]

Many thanks.

[Piltdown Man]

Cutting to the chase, this question is really about what happens to your IAS as you climb with a constant mach number. And remember, don't let reality get in the way of an exam answer.

PM

[keith williams]

The climb up to the crossover altitude is carried out at constant IAS.

This means that the TAS is increasing, so the aircraft is actually accelerating in the climb.

So the available thrust is being used for three things.

1. To balance the drag to prevent deceleration.
2. To support some of the weight in order to push the aircraft up the climb path.
3. To provide thrust in excess to drag to accelerate the aircraft to achieve the TAS increase that equates to constant IAS.

At the crossover altitude the climb changes to one of constant Mach number.

This means that the TAS is decreasing, so the aircraft is actually decelerating.

So not only do we no longer use some of the thrust to accelerate, but because we want the aircraft to decelerate (for constant Mach), we also need to devote less of the thrust to balancing the drag.

We now have more thrust availalble to support part of the weight, so we can achieve a greater climb angle and ROC. We do this by increasing pitch angle.

That is the theory of the subject.

The post by Georgeablelovehowindia appears to indicate that it is also the reality.

[Artisan]

Georgeablelovehowindia, Thanks for the tip Chum and yes I do enjoy sailing my yacht as much as possible.

I haven't flown any of those old classics, so I can't dispute your observations. My Real aircraft is the B737-800. After careful observation I can state that, at the changeover altitude; THERE IS NO PERCEPTIBLE CHANGE IN ROC OR PITCH ATTITUDE. That is a fact.

Below the changeover altititude, IAS is constant, TAS is increasing, Mach is increasing. ROC is generally decreasing, primarily due to the fact that the thrust force produced by the engines, at a constant RPM setting (Climb Thrust), is also steadily decreasing.

Above the changeover altitude, the transition to Mach is incredibly smooth, there is no change in pitch. TAS immediately starts decreasing, Mach is constant, ROC keeps decreasing as before.

[keith williams]

It is intersting, but not surprising that different aircraft respond in different ways. Things such as values of IAS and ROC will affect the rate of change of TAS as we climb and this will affect the amount of thrust that is being used to accelerate at constant ISA below crossover. The greater the ROC at crossover, the greater will be the (predicted) increase in ROC as we shift to constant Mach.

The sensitivity of the autopilot will also have an effect.

We probably should't expect an abrubt change in anything.

During the climb up to crossover, the autopilot was presumably gradually reducing pitch in order to maintain constant ISA. Was this gradual reduction detectable? If it was not, then we should not be surprised if the gradual increase in pitch above crossover is also not detectable.

[Artisan]

Pitch attitude was gradually but noticeably decreasing during climb below changeover altitude. Above changeover altitude pitch continued to decrease as did ROC.

[Denti]

It happens on the 737 same as on those aircraft george mentions. Climb rate will slowly increase for some time after changeover altitude before it decreases again. It was more noticeable in the 737 classic where climb rates in excess of 2000ft in the 300s were not uncommon and seem to be less obvious in the 800 while still being the same in the 700.