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"On-Approach" Logic on STAR?

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Moderator
Registered: May 2009
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I've learned that the allowed speed "flexibility" is -10/+15 knots when above speed transition altitude, else -10/+10 knots.

-16 knots sounds a bit critical to me. Is this value from an official source, Jon? You could drop into the lower part of the lift/drag curve and so the conditions regarding recapturing the path from below could get even worse. Additional drag is fine when above the path, but not when below the path, I would say.


|-|ardy



Britjet wrote
There seems to be two aspects to this, apart from the approach logic discussion, which I think we all agree on. They are whether we should see THR or SPD, and whether we should see VNAV SPD or VNAV PTH. One caveat is we should never see SPD-VNAV SPD!

I agree ...
Member
Registered: Aug 2014
Posts: 375
Location: Camberley, UK
emerydc8 wrote
Yes, but SPD-V/S will not cause the wings to fall off the airplane like some guys fear!


Of course not, Jon, that is very silly of them. The THR button does that..

Peter
Member
Registered: Jun 2015
Posts: 103
Location: Tucson, AZ USA
Quote
-16 knots sounds a bit critical to me. Is this value from an official source, Jon?


Yes, Hardy. This is right from my airline's systems manual (Vol. 2) and it probably came from Boeing.

The speed increase from FMC-target speed to stay on path is up to Vmo - 16 (349 knots). This gives a lot more ability to stay on path. You shouldn't see the DRAG REQUIRED scrathcpad message until it reaches 349 knots and it still can't keep it within 150' of the path. That's when speed reversion occurs and it goes to VNAV SPD.

You run into the smaller deviations once you are below the first speed constraint (see page 2).

Quote
Of course not, Jon, that is very silly of them. The THR button does that.


I try to tell them that!
Moderator
Registered: May 2009
Posts: 5140
I mean minus 16 sounds critical. Plus 16 is OK.

DRAG REQUIRED relates to the positive side.

I agree that +16 is correct.

-10 and +15 are the limits.

+16 is an exceedance of +15 :-)

Below 10000 feet +15 becomes +10, i.e. +11 is then the exceedance. You shouldn't exceed 250 KIAS below 10000 feet.
Member
Registered: Jun 2015
Posts: 103
Location: Tucson, AZ USA
Just so we're on the same page here (page 1), the 16 refers to Vmo minus 16 (354 knots) which is how fast it should be able to go to maintain the path if you are being pushed above it on descent. The 15 refers to 15 knots below target FMC, which is how slow it should be able to go to maintain the path if you are falling below it.
Member
Registered: Jun 2015
Posts: 103
Location: Tucson, AZ USA
Hardy Heinlin wrote
I mean minus 16 sounds critical. Plus 16 is OK.

DRAG REQUIRED relates to the positive side.

I agree that +16 is correct.

-10 and +15 are the limits.

+16 is an exceedance of +15 :-)

Below 10000 feet +15 becomes +10, i.e. +11 is then the exceedance. You shouldn't exceed 250 KIAS below 10000 feet.


I think 10,000 would normally be the first speed constraint unless there is a speed constraint higher than 10,000 -- like on the LENDY 6 (250 knots at LENDY). After that point, it should go to the second set of rules on page 2. So, 10,000 would be the speed transition altitude, but not necessarily the speed constraint point.
Member
Registered: Aug 2014
Posts: 375
Location: Camberley, UK
Just to be clear...the speed does not increase to 349 every time and then a "drag required" message appears. That's daft. The only time it will fly that fast without giving you a message is if you put a very high speed into the FMC.

The accepted + increments apply on whatever speed you put in the FMC. NOT 349 kts blindly...no way...

(Descent at 350 kts is crazy anyway IMHO)

Peter
« Last edit by Britjet on Sun, 02 Aug 2015 21:54:43 +0000. »
Member
Registered: Jun 2015
Posts: 103
Location: Tucson, AZ USA
I'm having trouble with that one. If you put 280 knots in the FMC, then got pushed high prior to the first speed constraint, when would you get the DRAG REQUIRED message and how fast would the aircraft go to stay on path?
Member
Registered: Aug 2014
Posts: 375
Location: Camberley, UK
10 kts until you get the message, Jon,

You aren't seriously trying to tell me that if you put 280 kts in the box it won't tell you that you need drag until you get to 349?

Peter
Member
Registered: Jun 2015
Posts: 103
Location: Tucson, AZ USA
Where are you getting that 10 knot figure?

I'm just reading from the FCOM:

"[W]ith greater than 349 knots (VMO/MMO minus 16 knots) the scratchpad message DRAG REQUIRED displays. The airplane may accelerate up to 354 knots (VMO/MMO minus 11 knots) to maintain the path."
« Last edit by emerydc8 on Sun, 02 Aug 2015 22:08:41 +0000. »
Member
Registered: Aug 2014
Posts: 375
Location: Camberley, UK
From your own text, page 2, about para 3 ( sorry, I don't seem to be able to copy and paste).

10 kts for the message, 15 kts for the allowed speed deviation.

I have seen the 349 kts text as well. Like a lot of Boeing publications the context can be lost or misunderstood. I don't think ( in fact I know tbh) that this is not the general case.

When did you ever fly a 747 that let the speed get to 349 kts before it gave you a drag message?
Member
Registered: Jun 2015
Posts: 103
Location: Tucson, AZ USA
What you are referring to on page 2 is in contexts where you are below the first speed constraint. If you are above it, I think page 1 applies and the 349/354 would be used.

I'm trying to find anything in the manuals that would controvert page 1 because, like you, I too think I have seen the DRAG REQUIRED message while above the first speed constraint and I was not doing 349 knots.
Member
Registered: Aug 2014
Posts: 375
Location: Camberley, UK
I understand what you are saying, Jon, and I understand the text.
But in my experience, it doesn't happen.
There is no way that you put 280kts in the box, for example, and it gets to 349 on its own...

I don't think I have ever done 349 kts in my life for real in any case - (the wings fall off, as you know) but I have seen plenty of DRAG REQUIRED messages...

Peter
Member
Registered: Jun 2015
Posts: 103
Location: Tucson, AZ USA
I agree. In practice, I think it would take one hell of an unforeseen tailwind to even get close to requiring 349 knots on a descent. I'm thinking of a situation where ATC keeps you high and you are finally released for the slam-dunk arrival. Does it wait until 349 knots to give you the DRAG REQUIRED warning? I think it happens before that. But the manual is contra. Sure wish we had access to that sim!

Come to think of it, if ATC kept you high or you just didn't reset MCP altitude and you were in cruise (M.84), I think you would eventually get DRAG REQUIRED at some point. Thoughts?
Member
Registered: Jun 2015
Posts: 103
Location: Tucson, AZ USA
Okay, I found this in the Big Boeing Book. I don't know if it applies to the 744, but it does tend to support your position that it will only descend up to a certain speed above the FMC target speed to stay on path.

I'd sure like to learn what Boeing meant by its reference to the 349 knots to set off the DRAG REQUIRED and up to 354 to maintain path. Above the first speed constraint, it makes no reference to the FMC target speed when the aircraft is being pushed high -- just that it will vary the speed up to 354 knots to maintain path. Do you think this could have been a software option or a later change?

Quote
DES

LATE DESCENT

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 REQUIRED 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 10 kts (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.
Member
Registered: Aug 2014
Posts: 375
Location: Camberley, UK
I don't know, Jon, but the quote above makes sense to me...
Peter
Member
Registered: Jun 2015
Posts: 103
Location: Tucson, AZ USA
Morning, Peter. I have been researching this issue for the past ten hours. I think the DRAG REQUIRED message comes on even if you are on the path but just fast.

Example:

You are level at 15,000' doing 300 knots in |SPD|LNAV|VNAV PTH|. You are 5 miles from a fix that has no speed restriction associated with it. Then, ATC instructs you to cross that fix at 220 knots.

If you insert that 220 knot speed restriction into the LEGS page and execute the throttles will go to idle and I will bet money that you will get a DRAG REQUIRED message.
Moderator
Registered: May 2009
Posts: 5140
If a 100-page document says ...

"DRAG REQUIRED appears when X is true"

... then such a sentence just mentions one condition. You cannot know if X is the only condition, or whether there are additional conditions A, B, C, D ... mentioned somewhere else on those 100 pages, in different words or in a less vague language, or even with contradictory details.

There are zillions of function triggers in the avionics of this aircraft. Nearly every function is not just triggered when a target value is exceeded; a trigger typically also involves:
- a time delay
- a hysterisis
- a trend

Without these additional interactive conditions, you would get lots of nuisance messages or flip-flop effects.

For example, I bet that the DRAG REQUIRED message will not be triggered only when a speed is exceeded; there must also be a time delay that allows short-term fluctuations and/or a hysterisis to inhibit flip-flops, and a trend away from the target.


Cheers,

|-|ardy
Member
Registered: Jun 2015
Posts: 103
Location: Tucson, AZ USA
Hi Hardy. I agree that there is obviously more that will set off the DRAG REQUIRED message than just being unable to stay on a VNAV PTH descent at 349 knots. I'm just trying to get something in writing on it, but Boeing seems to have decided that no one needs to have that information.

I can find only two places where DRAG REQUIRED is mentioned in my 744 manuals. One is the two-page document I presented above. The other is under FMC scratchpad messages, where it defines the DRAG REQUIRED message as "VNAV active and additional drag required or autothrottles off and less thrust required to maintain descent path."

It must make it difficult to write an accurate simulator program without access to it. Do you have access to this information?

Jon D.
Member
Registered: Jun 2015
Posts: 103
Location: Tucson, AZ USA
Hardy Heinlin wrote
If a 100-page document says ...

"DRAG REQUIRED appears when X is true"

... then such a sentence just mentions one condition. You cannot know if X is the only condition, or whether there are additional conditions A, B, C, D ... mentioned somewhere else on those 100 pages, in different words or in a less vague language, or even with contradictory details.


True, but when we see RESET MCP ALT on the scratchpad, is there any other place -- other than 2 minutes prior to top of descent in VNAV -- that we should expect to see that? I don't think so. Somewhere, there is documentation of what other conditions should trigger the DRAG REQUIRED message. I'd like to find it.

[ADDENDUM]:

The two pages I provided above came directly from my company's FCOM, which I presume they got from Boeing. I did a word search of the FCOM for DRAG REQUIRED and other words that might turn up any discussion of this message. So, if Boeing is trying to hide it somewhere using different words, they are doing a good job of it!
« Last edit by emerydc8 on Tue, 04 Aug 2015 00:02:19 +0000. »
Member
Registered: Dec 2009
Posts: 334
Location: KTPA
According to "the Man" Bill Bulfer there are only 3 scenarios that result in the DRAG REQUIRED message...

DRAG REQUIRED (AL/4)
VNAV engaged and additional drag required or A/T off and less thrust required to maintain descent path.

DRAG REQUIRED (AL/7/g)
Airplane is unable to maintain the precomputed nominal descent path and stay within speed tolerances

DRAG REQUIRED (AL/n/p)
VNAV engaged and additional drag required to track descent path and maintain command speed.

I quote from page 257 of his Big Boeing FMC User's Guide.... so Instance 2 or 3 would seem to indicate that you WOULD get this message even when on path, but fast....

HTH- C
Member
Registered: Jun 2015
Posts: 103
Location: Tucson, AZ USA
The problem here is that only the first instance (al/4) clearly applies to the 744. It looks like the second (al/7/g) applies to the 777 Pegasus; and the third (al/n/p) applies to the 757/767 and non-PIP aircraft. I don't know whether the third would apply to the 744, but even if it did, it's still only covering the descent -- not level flight. It doesn't shed any more light on when exactly the DRAG REQUIRED message will appear -- specifically what triggers it.

KEY
Alerting AL
Advisory AD
Entry Error EE
Comm C
all all versions of software
n Non-PIP
P PIP 757/767
g Pegasus
4 747
7 777
AAA VOR identification
AAAA LOC identification
NNNN Numerical value
XXXX Alphabetical value
These codes are not 100% accurate.
« Last edit by emerydc8 on Mon, 03 Aug 2015 23:41:09 +0000. »
Member
Registered: Dec 2009
Posts: 334
Location: KTPA
Jon-

I've copied Bill and asked for his thoughts... will advise as soon I hear back.

Best- C
« Last edit by cavaricooper on Tue, 04 Aug 2015 00:09:36 +0000. »
Member
Registered: Jun 2015
Posts: 103
Location: Tucson, AZ USA
Thanks. Not to beat a dead horse, but it seems that everyone has an opinion about when the DRAG REQUIRED message is triggered; and I haven't seen anything in writing other than the 349 knots (VMO/MMO-16), which I presume came directly from Boeing. I know there's got to me more because I think everyone who's flown the airplane recalls seeing it when they were not even close to 349 knots.

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