Speed Control (again)

[zonoma]

This is part question from me and part awareness for everyone to do with Area Speed control during early descent phase of flight.

A collegue cleared a british owned Bus to (exact words) :

"when ready descend FL*** LVL (fix) speed when able 280kts"

half way thro the descent we spotted, using our groundspeed readouts, that this certainly wasn't being done so queried the pilot (confirm current speed) who replied "currently 340kts, reducing 280 as requested". Too busy at the time to discuss any further details as to why, and I do understand the whole Mach v IAS/transition across thing, but surely as the IAS increases during initial descent on a Mach, 280kts would not be exceeded at any time due to the restriction????

Q. Once 280kts intercepted, can this be maintained all the way down?
Q. Concerning the 'normal' transition level for speed, would you amend it for the purposes of speed control?

In this case, fortunately the controller also had headings in force to ensure separation, but if not, a Frenchman wouldn't have been too happy either, this letting us in on the others' speeding antics. However it is scary that an approved form of separation can be meddled with in such a big way. Yes, I don't fly yours or any other aircraft, but I understand enough to get along nicely. How many understand the measures of separation that we can use, and identify them as soon as they are issued to you in the cockpit?? If this pilot had listened, he would have heard in successive transmissions the frenchman get 300kts or more, himself get 280kts, and the Italian just behind get 280kts or less. This would have put them all nicely 10nms in trail, and possibly a straight off stack and into land (wasn't all that busy at LHR) rather than the 10nm already acheived being eroded till both were side by side, eventually giving the Brit a spin for spacing.

We don't do it for laughs, and are much better at it than ever before due to Groundspeed readouts (yes, something that actually works to advantage at Nerc), but maybe I'm still missing something?? :confused: :confused: :confused: :confused:

[411A]

Have never been able to understand WHY, when the controller asks for a certain speed (within reason) pilots are not able to "just do as they are told". Hey guys, it ain't all that difficult.
Cooperate and graduate.
Stop making excuses that..."...well at flaps 10 our scheduled speed is..., or...."well this is not in keeping with our SOP's..."

Wake up! There are OTHER flights inbound other than yours, do as you are TOLD. IF you cannot, don't bother to call yourself professional.

[NW1]

Could it be possible that the phrase "speed when able" may have implied (rightly or wrongly) some discression? Maybe a more explicit clearance like "descend at Mach 0.8 then 280kts" might have helped nail it down?

I agree with 411A's meaning (though not his agressive style) that clearances should be complied with expeditiously (they are, after all, given for the best reasons!!) or else warning given immediately non-compliance is unavoidable, but very often communication problems can ocour in spite of relatively tight RT standards. I think that may be what happened here.

[expedite_climb]

I would have interpreted this as to descend at a rate which would mean you could make the fix by the FL, and then when you can do that slow to 280 kts.

If you reduced to 280kts immediately you might not make the fix at the level.

If 280kts is maintained all the way down this may cause probs if the crew had planned on a higher speed as 280kts would leave them high.

[zonoma]

As it was a "when ready descend " clearance, and the a/c didn't vacate for about 20nm, the speed/level-by problem should not apply. If this was the case, then the pilot certainly should inform us of the 'not able to comply' with an executive instruction, be it either level-by or speed.

NW1, I think that the phrase used was, without doubt, the correct one to use, as speed when able means just that and not when you want to. The phrase you suggest would give the exact problem we have here, whereby some decide to stay on Mach number until passing say FL200 giving a huge IAS to reduce from, rather than switching around FL300 when 280kts is acheivable.

Other threads recently (and not so) have banged on about wordage for speed control, and the one used is direct and unambiguous, the other ones used are :-
"when you switch to indicated" - gives the problem highlighted above.
"on passing FL280, IAS ***kts" - the one we are supposed to use, but we're not pilots and too many times get response, "can't do that until FL***" Too much RT. Also some are already doing 300kts+ by this level, and thats unwanted.
"descend FL***, IAS ***kts" - response, "not able that speed at the moment". We do know that, but its the speed we want acheived, but again this phrase can get unwanted replies.
"descend FL***, make the speed transition 280kts and maintain for descent" - have only ever used this twice, first response "eh?????", second was unpleasant as I got the impression he thought I knew/was attempting too much, and we also don't know the handling of every aircraft on every different day.

Its a difficult battle to get speed control affected, and the RT guidelines poor. One day someone will come out with the knockout phrase that combats every problem, and ICAO hopefully snap it up immediately, lets just hope its soon :( :( :(

[411A]

...in the meantime, pilots should (within reason)"fly the speed assigned" .....period.

[NW1]

"Descend to FL140, level 40d OCK, maintain M.80 then 280kts. Do not exceed 280kts indicated"?

If he fails: "Maintain FL240, hold BNN, delay 45 minutes. No speed control."?:)

[expedite_climb]

Okay, just re-read what you wrote. I think I would have been confuzzed and clarified it !:D :D

[NW1]

Okay. Did as you asked. Clarification as requested:

"Descend to FL140, level 40d OCK" Easy bit. Sets a scene and all that. Good instructional technique:D.

"maintain M.80", still easy, no? M0.80 (or whatever this Bo' or Airbus is dragging along at) initially in the descent....

"then 280kts." Getting more complex, but M0.80 at (say) FL370 is about 260kts indicated. A constant Mach descent will result in an increasing IAS. This part therefore means trap 280kts when you see it (FL340-ish) and don't just maintain M0.80 allowing the IAS to go up to 350kts prior to reducing back to 280kts later on 'cos you'll bump into someone. The "then" bit is to emphase that you're not to go for 280kts immediately because in this case that would involve a 20 kts acceleration to about M0.85 (and a compromised separation) at FL370 rather than a M0.80/280kts descent profile (the desired result).


"Do not exceed 280kts indicated" Back to easy, really. Take the occasional look at the ASI, and irrespective of Mach don't let the big hand point to more than 280 at any stage. Sounds obvious, but it seems it wasn't to this bloke.

Well, it was only my go at putting it into a short instruction that even the special needs pilot quoted by zonoma would be able to comply with. Look forward to your improved suggestion.......:)

PS: All meant a little tongue-in-cheek, just in case anyone felt like a pprune-type rant..........

[expedite_climb]

Thanks NW1,

What i meant was i did understand it 2nd time round, but if id been flying and given the instruction i would have had to pause for thought and maybe ask for clarification.

I suppose it reality it would be as simple as adding the restriction in the fmc, and changing des page to 280/.80

Good explanation though... :-)

[NW1]

Yes, it is an easy thing to do whether you're FMC equipped or not, the difficulty was putting it in a short R/T instruction which left no room for "personal interpretation". I think "Mach X THEN IAS Y" should do it - but from the above it would seem the need to switch over to "IAS Y" when you reach it and not just go tanking on at "Mach X" needs emphasising...... I agree with you, though - it shouldn't!

[Young Paul]

411A: I don't have an axe to grind, but I'd like to know how many of your 1500+ posts start with "I've never been able to understand why...." or similar. But then, not many Americans understand why Bush's words about the evil dictator holding the world to ransom with threats of force should be viewed as so ironic by the rest of the world either ....

The bottom line is (speaking as a pilot) that the pilots were wrong. ATC, your apprehension is entirely correct, and the normal understanding of pilots is precisely as you say - constant Mach until 280 knots is reached, and plan the descent to make the profile work on this basis. It sounds like they were trying to pull a sly one.

However, I was "cut up" very last night by a 777 sliding over the top of us on the way into BNN at high speed and then being descended 5 miles right of us and about 0.5 miles ahead on the intermediate approach, so it's not only pilots who adjust things and expect people not to notice. (Though it's quite nice to have 5 miles of space in front of you on final for 27R :) )

The only thing I would say is, if you want us to make a level restriction that is unexpected, let us know before TOD. If you want us to make a level restriction and a speed restriction, let us know before we ask for descent.

[A-V-8R]

Jeez, I hate to be flamed. 52 year old man with already burning hemmorroids.

If you issue a speed, I think it must be complied with.

The problem as I see it is with what altitude one transitions to IAS versus mach.

My aircraft transition via the FMC at FL 260. It is a Boeing product. God knows whatever the Frogs are doing.

If you assign a speed for the descent, then give a transisition FL to switch to IAS from Mach.

I re-read the FOM pages every time I fly, but it seems to change all the time, depending where I am going.

No book in front of me at this hotel, but in the US I think the FL is 310 to assign speed in IAS versus Mach....(Going down!)

I think that the transition altitude (Not altimetry) for Mach versus IAS is 260, in Europe, different in other areas of the world...

I'm no longer in the building flight time mode. The Barber pole suits me find for descents..........

[flufdriver]

Iknow that this probably does not apply in this case, but it also happens that ATC is only human just like Pilots, therefore when I am being given a partial clearance or instructions without explanation (i know this because I frequently fly the same route) then it is possible that i might push the envelope because i don't like to be the patsy all the time.
For example: If AtC of a certain area that is dominated by one large carrier issues instructions to us like: After passing xxxxx fly hdg 340 for entrail spacing and then the guy 4000ft above and 10 miles behind us with the two big AA's on his tail, is cleared direct to XXXXX descend PD to cross XXXXX 10000 @ 250kts then i know that the explanation for my instructions was something like: you are #2 behind the guy behind and above you, his company does not like him to be late and they have a hell of a lot more clout than you little guys do.

[5milesbaby]

fluf, none of that over here, too much work on our behalf for a start.

A-V-8R, we aren't pilots therefore cannot really give a transition altitude, so we expect for you to work the altitude out for yourselves with the speed given, ie transition altitude = when you intercept the IAS given. Realistically is that a problem, or can it be done easily??

[West Coast]

Fluf
I feel your pain. Next time into DFW, "American tower xxxx on a 10 mile final"

[A-V-8R]

Sure you can issue a transistion altitude...but don't confuse it where we reset the altimeters....

Here in the states, the ATCcontrolers manual sez use Mach technique FL310 and above and IAS below FL 310.....

If you are a controller, issue a clearance such as.....

"UAL 976, Descend to FL 270, Maintain .82 until FL 310 then 280 IAS......."

In whatever city/state you are in, I'd believe the Controllers handbook would define speed technique as in the described in Paragragh 2 above.....although the FL's may be other than FL310 in other parts of the world......

[Grotehaasje]

Going back to NW1's original point - it isn't a problem in a modern flt system to transition from a Mach No to an IAS.

The pilot can put the 2 values into the FMS as a Tactical mode, or can use the flt system. For example, on the F100 selecting Mach Hold on the glareshield then pre-setting the desired IAS will ensure that the flt system switches from one to the other, and maintains, at the appropriate point.

I think the Bus works in a similar manner.

[vector4fun]

"If able, reduce speed to 280 kts, then descend to FL****"

"If Un-able, turn left sixty degrees for spacing..."


Works nearly every time.
:D