"Transition Speed" from a newbie

[greenasgrass]

Heard the other day a guy asked his current speed. He indicated ".79". He was told to "transition at 250 knots". So, being new to the whole IFR thing, could someone enlighten me as to what that is, or what is expected??

Thanks,

Green as Grass...

[AuxBatOn]

I think it would mean to hold Mach 0.79 until he reaches 250 kts (until Mach 0.79 gives him 250 kts, which should happen around 36000 feet), then maintain 250 kts. Probably in the descent.

[pika]

The speed where you transition from mach number to IAS in descent. In a descent IAS will increase while maintaining a constant mach number so when the desired transition speed (IAS needle) increases to the .79 needle consider yourself transitioned so now fly IAS. Usually taken care of with the FMC so you still have enough time to finish your coffee in descent. Generally speaking .80/300 is a common practice.

[No_Delay]

The above definitions are for the most part correct. Read up on Wikipedia about true airspeed versus indicated versus mach for an in depth answer.

From a controllers perspective, If you have a mach restriction on an aircraft, once issued initial descent clearance at destination, the restriction, technically is thrown out the window. The pilot is free to speed up / down as he pleases unless you specifiy the restriction in effect during descent. Next the mach restriction is again thrown out when the aircraft 'transitions' to indicated speed.

So if I am running them tight in descent I will assign a mach # and a transition speed.. This leaves nothing to chance and allows you to execute your plan for sequencing.

[Rockie]

Usually taken care of with the FMC so you still have enough time to finish your coffee in descent. Generally speaking .80/300 is a common practice.

The first sentence is usually wrong because of the second sentence. Descent in jets is normally around .80/300 at idle thrust to 10,000 ft then slowing to 250. When ATC issues a transition restriction it's usually very close to, or even after your descent point. The FMS will recalculate your descent at 250 kts which of course means you are instantly way too high given the resulting lower rate of descent. The FMS will then abandon the descent path and descend at 250 kts. In other words, you have to manage your own descent because the FMS just gave up. You then have to put down your coffee and fly the airplane.

[greenasgrass]

OK, let me see if I have a grasp on this, or not.

I'm zingin' along at 39000' indicating Mach .78 on outer scale, which corresponds to, say, 220 knots indicated on the inner scale. The guy on the ground, without the view I have, says"transition at 270 knots". So do I correctly understand that I can maintain this mach .78 only until such time as 270 knots and it coincide? Hence afterwards I maintain only 270 knots, even tough the speed I want to descend at is increasing away from me?

Hope this makes sense...

GaG

[Rockie]

Correct. ATC does that to maintain separation between arriving aircraft. The smart crew listens up to what's going on around them and plans their descent with that in mind in case they are forced to slow down. It just means starting your descent a little earlier.

[V1RotateV2]

There is also another situation:

The crew will inform their possible or intended speed as "C-GXXX doing 255 transitioning to 280 below FL200". They anticipate to the controller a change in indicated speed.
The controller may come back with "C-XXXX transition to 260 or higher when possible".

This usually happens because some a/c have a maximum indicated speed restriction that changes with altitude. Especially true in turboprops.

[pika]

pika wrote:
Usually taken care of with the FMC so you still have enough time to finish your coffee in descent. Generally speaking .80/300 is a common practice.


The first sentence is usually wrong because of the second sentence. Descent in jets is normally around .80/300 at idle thrust to 10,000 ft then slowing to 250. When ATC issues a transition restriction it's usually very close to, or even after your descent point. The FMS will recalculate your descent at 250 kts which of course means you are instantly way too high given the resulting lower rate of descent. The FMS will then abandon the descent path and descend at 250 kts. In other words, you have to manage your own descent because the FMS just gave up. You then have to put down your coffee and fly the airplane

The FMS will not recalculate a descent at 250 knots due to not descending when initially predicted. Show me a reference for that and I'll change my opinion.

[ScudRunner]

lol you guys and your FMS's doing all the work back in my day (1 month ago) on the 727 its all calculated in the grey matter.

[hamstandard]

lol you guys and your FMS's doing all the work back in my day (1 month ago) on the 727 its all calculated in the grey matter.

Lets see, 3 to 1 because we are at max landing weight straight in with no wind or we are light so I will shave off 10 miles and adjust 10 miles for every 50 knots of wind. But then there is the lateral manouvering so I have to take that into consideration, and the there is visible moisture requiring engine anti-ice so I will have to start down earlier but how thick is that cloud I wonder and at some point it will warm up. Oh who cares, You are never too high in the old 727. Speed brakes, Gear and eventually flaps.

[Rockie]

pika wrote:
Usually taken care of with the FMC so you still have enough time to finish your coffee in descent. Generally speaking .80/300 is a common practice.


The first sentence is usually wrong because of the second sentence. Descent in jets is normally around .80/300 at idle thrust to 10,000 ft then slowing to 250. When ATC issues a transition restriction it's usually very close to, or even after your descent point. The FMS will recalculate your descent at 250 kts which of course means you are instantly way too high given the resulting lower rate of descent. The FMS will then abandon the descent path and descend at 250 kts. In other words, you have to manage your own descent because the FMS just gave up. You then have to put down your coffee and fly the airplane

The FMS will not recalculate a descent at 250 knots due to not descending when initially predicted. Show me a reference for that and I'll change my opinion.

I can't speak for your FMS Pika, but every one I've ever used has recalculated the descent path. It's just a fancy adding machine afterall. Nor can I give you a reference since no FMGC manual I've ever seen addresses specifically what happens when you change things mid stream like that. You'll just have to trust anecdotal evidence from hundreds of occurrences over many years of experience. Or not.

[pika]

Recalculating the descent path is one thing, recalculating the descent path at 250 knots is entirely another.

[Rockie]

That's what we're talking about isn't it? The FMGC recalculates a descent path at 250 kts to meet the first altitude constraint, either in the STAR, approach or the runway itself. Since the descent rate at 250 is much lower than what you had planned originally the new descent point will be much earlier than the high speed one, or much lower than you currently are if you've already started your descent. They calculate that to give you some idea of where you are in relation to where you should be if you want to follow FMGC calculated paths.

Since the airplane is now well off the calculated path, the airplane can't follow it anymore and will default to the new programmed speed for the descent (250 kts in this case). Now, if you want to meet the first altitude constraint you have to do something pilot like. Usually speedbrakes.

[pika]

Here's a bit about late descent. No mention of 250 knots.

"Depending on where the aircraft is in the descent, there are two procedures employed by the FMS in attempting to recapture the descent path.

First Procedure: Occurs prior to the aircraft reaching any descent airspeed or altitude constraint. When
above the descent path and prior to the first constraint, the aircraft will maintain the FMS target speed plus 15 kts as long as that speed is less than or equal to Mmo/Vmo. If this speed is greater than that, then the DRAG REQUIHED message is displayed.

Second Procedure: Occurs after the aircraft reaches the first constraint or is below the SPD TRANS altitude or is in Speed Intervention. In this procedure, the aircraft maintains the FMS target plus l0kts (or MCP speed if in S.I.), as long as that speed is less than or equal to Vmo. If this speed is greater than Vmo. DRAG REQUIRED is displayed."

[Rockie]

Like I said, I can't speak for your FMS. The 250 thing though was brought out as an example of changing your descent speed and having the FMS recalculate the descent path. In the quote you give there is no mention of 250 because you aren't recalculating anything. The original descent speed is still programmed. I could just as easily said 200 kts which would still serve the point I was trying to make, and it wouldn't appear in your manual either. Because it was just an example.

This quote doesn't explain what your FMS would do if you entered a different descent airspeed, and there is nothing in the quotes circumstances that would force the FMS to recalculate. Next time you're flying try it out. Wait until you're on the descent path and then change the FMS descent phase airspeed. I don't know what airplane you fly, but unless it's really rudimentary the FMS will recalculate the descent path and put you either high or low depending on what speed you put in it.

[yfly]

Usually taken care of with the FMC so you still have enough time to finish your coffee in descent. Generally speaking .80/300 is a common practice.

The first sentence is usually wrong because of the second sentence. Descent in jets is normally around .80/300 at idle thrust to 10,000 ft then slowing to 250. When ATC issues a transition restriction it's usually very close to, or even after your descent point. The FMS will recalculate your descent at 250 kts which of course means you are instantly way too high given the resulting lower rate of descent. The FMS will then abandon the descent path and descend at 250 kts. In other words, you have to manage your own descent because the FMS just gave up. You then have to put down your coffee and fly the airplane.

Yep, this is exactly what my FMS's will do. As a result, I let the FMS do lateral and vertical nav and select Manual Speed at 250. Then I am free to sip coffee.

[Rockie]

If you are descending on a flight path calculated for idle thrust at 300 kts then manually select 250 kts without doing anything else, you're still coming down that path at 300 kts. You could select 60 kts if you want and you're still coming down that path at 300 kts unless you increase drag to slow down. Simple physics.

Imagine you're sliding on a wire downhill and gravity gets you up to 100 kts. Wishing you were going slower (which is what manually selecting a lower speed is) doesn't make you slow down. You have to throw out the brakes.

I fly with guys all the time who are glued to the FMS, and have completely forgotten how to actually manage a descent. They do things like put out the speed brakes at FL330 on descent because the speed got a little high. Or accept crossing an altitude restriction 300 ft high because the FMS was slowing the airplane down at the same time as trying to descend and didn't quite make the calculation as accurately as it should.

Moral of the story: The FMS isn't as smart as you think it is, and it doesn't have a pilot's licence. Sometimes you have to put the coffee down and do the job you were trained to do.

[pika]

I'm going to the Flying Beaver to get two coasters. First one will cover the FMC screen and the second will be folder in half and placed under the bottom end of the flat coaster to even it out (just as most of the wobbly tables at the FB have). Then I am going to put my coffee on it and try that 3:1 thing the 727 boys are talking about.

[CAL]

go as fast as you can for as long as you can....18,000 for 250......
or if you are in the fmc /250 in the descent page in our machines...

[yfly]

If you are descending on a flight path calculated for idle thrust at 300 kts then manually select 250 kts without doing anything else, you're still coming down that path at 300 kts. You could select 60 kts if you want and you're still coming down that path at 300 kts unless you increase drag to slow down. Simple physics.

Imagine you're sliding on a wire downhill and gravity gets you up to 100 kts. Wishing you were going slower (which is what manually selecting a lower speed is) doesn't make you slow down. You have to throw out the brakes.

I fly with guys all the time who are glued to the FMS, and have completely forgotten how to actually manage a descent. They do things like put out the speed brakes at FL330 on descent because the speed got a little high. Or accept crossing an altitude restriction 300 ft high because the FMS was slowing the airplane down at the same time as trying to descend and didn't quite make the calculation as accurately as it should.

Moral of the story: The FMS isn't as smart as you think it is, and it doesn't have a pilot's licence. Sometimes you have to put the coffee down and do the job you were trained to do.

There is a lot I have forgotten over the years Rockie but operating an FMS and reverting to actually flying aren't included.

Your point is well taken but there is more than one way to skin a cat. Man speed and V/S works well as does resetting the FMS speed to 250 and selecting a vertical "Direct To" your first crossing restriction. Speed brakes as/if required. Even at my age I can manage that, sip coffee and even chew gum.

[Jug]

So... lets say I'm zipping along at FL410 doing .83, how do you figure out your transition speed in KIAS when ATC askes "C-GXXX, what will your transition speed be in descent today?"

[Steve Baker]

You can read it off the descent page on your CDU. A couple of people seem upset about the different ways to slow in the descent to meet an ATC instruction. FMC's all have similarities but, they have detailed differences. I find the discussion interesting but, it would be better if you gave the model of aircraft you are talking about when you explain the different techniques you use.

On the 737 ng, you have a couple of different options. Normally, if assigned a speed restriction after top of descent, you first go level change and set the new speed on the MCP. You can then change the descent speed in the FMC and you will get a new vertical guidance path to try and meet. Another way would be to enter the new speed restriction in the FMC and select VNAV speed vice path. Regardless, your going to need drag. The problem being on the -800 at least, the boards aren't very effective at 250 kts. If you have to slow a lot from the original plan or, if you don't have alot of altitude before the next restriction, you're back to ATC telling them they can have the speed or the crossing, not both.

Of course, just as previously mentioned, that's why it's nice to listen to what the guys ahead of you have been getting.

Steve

[swordfish]

Ok guys. I have read all the posts here, and I have a few ("dumb") questions. Please sit back relax, take a valium and then reply...

hamstandard:

we are light so I will shave off 10 miles and adjust 10 miles for every 50 knots of wind.

(a) I don't understand why you would shave 10 miles off when you have to slow down at 10,000 to 250. Seems that will lengthen your descent, not shorten it.
(b) The distance you adjust for winds - is that based on a certain mach/IAS? i.e. will it change if you're given a different transition restriction?
(c) If I fly a slower a/c (turboprop), would you recommend interpolating the wind adjustment to lower values? (since I don't see 50 kt often)
(d) I normally calculate the descent at 2000 FPM, and add a minute to 1½ min for slowing up. e.g. from FL240, 13 minutes back. The short time wind affects me appears to be insignificant, and also, I get a push at higher altitudes, which runs out between 10,000 and ground (or the other way round, of course, when descending from the East!). Hardly anyone else uses this method. What are the shortcomings or fallacies of this plan? i.e. why don't you guys use the same plan? It seems simpler to me, and works pretty well.

v1rotatev2:

Especially true in turboprops.

Yes, I have noticed this change in Vmo in all the turboprops I have flown, but I have never really understood why. Can you explain please?

rockie:

Sometimes you have to put the coffee down and do the job you were trained to do.

(a) Not many of them have been trained like this these days. My copilots all apply the 3 in 1 rule, and we end up wasting time, fuel, and arriving at initial altitude way too early. (Additionally, I don't permit coffee on the flight deck, as it invariably ends up on the fd, autopilot, or floor...) How can I counter this problem?
(b) why is it rockie, and not rocky?

yfly:

Even at my age I can manage that, sip coffee and even chew gum.

(a) It begs the question of how old you are, yfly?
(b) Does that work for iced tea also?

[Rockie]

Swordfish

a) I'm not sure which problem you want to counter, over-reliance on the FMS or spilled coffee?

b) I don't know why I spell it that way. It just came out like that when I typed it.