A320 Mach/IAS relationship to Rate of Climb/Descent

[Macarto]

Good Day Gentlemen,

I've two questions regarding the A320.

1) At high altitudes (>28'000), Airbus adopts a Mach Number instead of IAS. In a climb, I have observed that if a Pilot selects IAS during climb, his ROC will decrease. However if a Pilot selects Mach Number instead on his FCU, his ROC will increase.

Similarly during Descend, when Mach Number is selected ROD will increase and vice versa.

Can anyone shed light on the matter as to why this is so?

2) For a Night flight, I understand that we cannot rely on the results of the weather radar to observe for moisture in the atmosphere. Wing lights and beacon lights reflections help us verify that we are flying through clouds. However if all fails, and when Icing conditions prevails at TAT +10 to -40, is it safe to say that we should have Eng and Wing anti-Ice since we do not have much visual cues at night?

Your answers and advice will very much aid my learning process.

[Derfred]

1. As you climb at a constant Mach number your IAS decreases. What happens when you decrease IAS in a climb? The aircraft pitches up to fly the IAS, hence increased ROC. Vice versa for descent.

2. Use common sense to determine if you are in icing conditions. If that means turning on a landing light, turn on a landing light.

[ACMS]

Like he said. It's the relationship between IAS, MACH and Temperature with Altitude.

If you held 300 KIAS all the way up to FL390 your MACH number at FL390 would have increased to .95 which is way above MMO. Apart from the fact that is above MMO we know that the thrust available isn't enough for the Aircraft to go that fast and still climb. ( no excess thrust available ) therefore the ROC will decrease as your FL and TAS and MACH increase.

There is therefore a point where the TAS is around cruise speed and the Aircraft still has excess thrust available to climb up to optimum altitude. That point is around FL310 somewhere.

At the change over point 300 KIAS .81 KTAS 475 your ROC might be reduced to 700 fpm. Then you hold .81 the IAS will slowly decrease ( the pitch attitude increases slowly ), the ROC increases initially up to say 1,200 fpm then slowly reduces as excess thrust available reduces and the KTAS reduces.

What I'm trying to say is that you cannot hold 300 KIAS all the way up as it's going to be way too fast and you won't have enough thrust to keep climbing anyway.

Watch the TAS in climb, it increases up to the changeover point from IAS to MACH ( around FL310 ) and maxes out around 475KTAS or so then decreases back to around 465KTS in cruise.

Reverse happens in descent obviously.

Incidentally the fastest TAS your Aircraft can achieve is at the changeover ALT, that's where VMO and MMO are the same. Fly that speed and that's a fast as she goes.....

On the A330 that's 330 KIAS .86 at FL 300 around 507 KTAS. ( assuming ISA )


Checkout this website that has calculators for MACH KTAS etc.

http://www.hochwarth.com/misc/AviationCalculator.html

[mustafagander]

Quick and dirty answer - climbing with constant IAS means TAS is increasing, ie you are accelerating the aircraft, hence lower climb rate. Climbing with constant mach means you are decelerating in TAS terms. The performance guys decide what IAS and mach to fly and there is a cross over altitude where, on climb, you attain climb mach and fly that while the opposite happens on descent - you fly planned mach until you reach planned IAS then fly that.

As for icing, wing TAI is rare in my brand of jet above about A100. Engine anti ice should be treated as just that - ANTI ice, so get it on early when icing conditions exist. Bent fan blades are so expensive! Landing lights give you an idea of vis and hence potential icing. Individual engine types vary greatly in their sensitivity to icing - the old P&W JT9-3 wouldn't carry enough ice to chill your Scotch while the RR RB211 seems much more tolerant.

PS I fly big Boeings but I think the basics are very similar.

[Microburst2002]

As you know, as the mach number increases, compressibility effects become increasingly important, to the point that Mach number is what limits the envelope, so we fly with reference to it.

At constant IAS, TAS and Mach increase in a climb, so you are effectively accelerating. A part of the thrust from your engines is used to accelerate and not to climb. When you switch to constant Mach, IAS and TAS decrease. Now not only you are using all of the excess thrust for climbing, you are also trading some speed energy for rate of climb. So you switch from a TAS increasing condition to a TAS decreasing condition: from acceleration to deceleration. Switching to MAch during the climb is like progressively selecting less indicated speed, knot by knot. Rate of climb will increase significantly and immediately.

When you have time, I suggest you find a performance book, for jets, and take a look at it.

[ACMS]

Isn't that what I said???

Does anyone actually read what people say anymore or do they just double up?

[Microburst2002]

In Spain we say: "Good things, if brief, are twice as good"

Here at PPrune is common place to read more than one explanation to one question, even if they are the same. It always adds something. For instance, if the next time Macarto climbs he notices how the speed target decreases knot by knot after the IAS to Mach change over, he will get a very intuitive idea of what is going on.

But, I admit it, I am not the inventor of jet airplanes performance.

[ACMS]

I did say that in the 4th para if you read my post.....

[Microburst2002]

It is somewhat difficult to read, to me. Too long for my taste, although everything you say seems correct, of course.

Still you don't say that it seems like selecting a lower speed knot by knot, which I think it helps a lot to visualize the effect from accelerating to decelerating.

And since I am in a free country I decided to write a short text explaining the subject. And I like it, and I think it will help Macarto. And now I am bored and that is why I write this post. Because I am free.

And bored

[ACMS]

Me too....:ok:

[Microburst2002]

Actually, right now I am not in a free country.

Porn pages are blocked. Hence the boredom.:}

cheers

[Macarto]

Thank you all for the explanation.

Thus on descent when flying at Constant Mach, IAS increases knot by knot due to the higher compressibility of air at lower levels, Rate of Descent increases as A/C wants to pitch down to increase IAS. When A/C reaches max TAS, change over altitude happens and the A/C switches over to IAS decreasing the ROD?

Do correct me if I'm wrong as what I've written is base on my understanding of all of your generous inputs.

[Microburst2002]

You nailed it!

That is why, if your mach and speed schedule for the descent is a higher than the usual mach number and speed, rates can be very high. Specially if weight is low. In managed descents it doesn't matter because the system will introduce a repressurization segment in which you descend with some thrust, so the angle and rate are not so large and there is more time to allow the cabin to repressurize in the time available till touchdown. However if yo descend with OP DES from 390, very light weight and high mach number, you will descend in no time and you might experience high cabin descent rates.:{

As a last note, bear in mind that above the tropopause OAT is basically constant, and therefore the relation TAS-MACH is constant (IAS still will change for constant MACH but the knot by knot reduction will be less and the change in climb performance less, too).

another one: the mach number for the green dot at the optimum altitude (about two levels below the max altitude) is about .69 to .71. So if you have to expedite climb at high altitude and your mach is above .69, you can rest assured that you will not get super stuck at green dot and low rate below the opt level (i've seen it happen very often). If it is less than .69, trading speed for rate can be treacherous. From lower altitudes, you can mentally calculate green dot speed at the target level. The excess knots you have will be the maximum you can trade for rate of climb in an attempt to expedite climb.
Remember that as long as you are with constant IAS you are actually accelerating in a climb. Do not reduce to green dot for expediting unless it is at low level and then you can resume your normal (Accelerated) climb. Green dot will progressively increase with altitude and you will find yourself in a bad situation if you try to expedite climb to cruising level by pushing the EXPED button.

[hikoushi]

Yes. That is why, if you pull an .82 "open descent" in an Airbus (330) at FL 410, it will gradually give you a steeper and steeper rate. As a point of care to your passengers, doing this while descending into a tailwind (or decreasing headwind) will cause the airplane to nose over even more. I have seen a 6000+ FPM descent inadvertently entered in this manner. Not at all comfortable to a sensitive person in the back. Managed descent is more comfortable in the Mach levels, especially with the wind condition mentioned.

[FCeng84]

I think it is a good idea for anyone who wants to understand the operation of an airplane to get a hold of that model's speed/altitude envelope. From a physics perspective the most interesting version (in my opinion) uses altitude as the vertical axis and true airspeed as the horizontal axis. Lines of constant VCAS and constant Mach provide the detail. At lowest altitudes, the max speed limit is usually a line of constant VCAS or possibly constant equivalent airspeed, KEAS. At high altitudes, most commercial transports are Mach limited. The corner where the limit changes from speed to Mach when climbing is called the Vmo/Mmo corner. The altitude of that corner will vary from model to model, but usually lies with the range of 25K to 30K feet.

When you understand what physics has to say about emergency descent along the Mmo line and the Vmo line and how you have to significantly shallow your descent angle/rate as you pass through the Vmo/Mmo corner you will much better understand the subject of this discussion.

[hikoushi]

Microburst2002, looks like we both made our replies almost simultaneously! And made the same point, to boot! Ha!

[Microburst2002]

Hikouchi

Yeah, I have a reputation for that, hahah

[Microburst2002]

FCeng84

Yes. Graphs are even better than formulas!

[MD80rookie]

This is what you are looking for, a general illustration of the relationship:

http://www.atpforum.eu/attachment.php?attachmentid=262&stc=1&d=1307172367