B747-400 VNAV PTH Descent Speed

[cf6-80c2b5f]

Hello,

My company's FCOM states that prior to reaching the first speed constraint on descent, the aircraft will descend in VNAV PTH and pitch down to as high as 354 knots (VMO/MMO - 11) to stay on path (at 349 knots the scratchpad will display DRAG REQUIRED). When even 354 knots won't keep it within 150' of the path, speed reversion occurs, the pitch mode goes from VNAV PTH to VNAV SPD, VNAV resets the target speed to 349 knots, and the scratchpad message DRAG REQUIRED displays again.

This seems like a really high speed to be flying to maintain the path and to give warning that drag is required. Was there a revision to the software at some point that allowed this increase, or am I reading this wrong? The Big Boeing Book mentions it will only go up to the FMC target speed + 15 in this situation and the DRAG REQUIRED will illimunate at that point. But it doesn't state whether this applies to the B744. Thoughts?

Thanks!

[JammedStab]

Originally Posted by cf6-80c2b5f

Hello,

My company's FCOM states that prior to reaching the first speed constraint on descent, the aircraft will descend in VNAV PTH and pitch down to as high as 354 knots (VMO/MMO - 11) to stay on path (at 349 knots the scratchpad will display DRAG REQUIRED). When even 354 knots won't keep it within 150' of the path, speed reversion occurs, the pitch mode goes from VNAV PTH to VNAV SPD, VNAV resets the target speed to 349 knots, and the scratchpad message DRAG REQUIRED displays again.

This seems like a really high speed to be flying to maintain the path and to give warning that drag is required. Was there a revision to the software at some point that allowed this increase, or am I reading this wrong? The Big Boeing Book mentions it will only go up to the FMC target speed + 15 in this situation and the DRAG REQUIRED will illimunate at that point. But it doesn't state whether this applies to the B744. Thoughts?

Thanks!

I believe it will create a path to target whatever speed you have entered into the VNAV descent page. So if for example, you manually enter an unusually high speed such as 345 knots as the desired speed(or your cost index is of such a value that this speed is used, if that can happen), I suppose if the winds are not as forecast, you will see at 354 a change to VNAV SPD as overspeeding prevention becomes a higher priority than following the path and the aircraft goes above the path in order to maintain a margin from maximum operating speed.

[cf6-80c2b5f]

Thanks for the quick reply. So, if I entered a descent speed of .83/300 on VNAV 3/3 and had very high unforcast winds on the descent, how fast could the aircraft theoreticaly go to stay on path and at what speed would I expect to see the DRAG REQUIRED message?

[Avinthenews]

From memory it will fly anywhere between 15 knots below programmed DES SPD to VMO -16kts and stay in PTH, outside that it'll either add thrust (SPD-PTH) or it'll say drag required prior to intervening itself and dropping from PTH to VNAV SPD as you said.

The -8 displays this band which makes me think drag required doesn't appear till the top limit is reached likewise 15kts slow nothing happens but at 16kts SPD activates.

[cf6-80c2b5f]

Thanks. That's close to what my FCOM is saying -- 349 knots (MMO/VMO - 16) for the DRAG REQUIRED message, then up to 354 knots (MMO/VMO -11) before it drops out of VNAV PTH to VNAV SPD and resets FMC target speed to MMO/VMO - 16.

It just seems like I have seen DRAG REQUIRED when I wasn't going this fast. Is there something outside of this specific VNAV PTH descent that would trigger the DRAG REQUIRED message?

(EDIT): Maybe I misunderstood you. Are you saying that you think the DRAG REQUIRED message happens when more than 15 knots above FMC target descent speed?

I have a friend who is a captain on the 747-8 at NCA. He often tells me about the -8 "anomalies." He thinks that Boeing pulled a lot of their software engineers off the -8 project when they had trouble with the 787.

[Avinthenews]

As long as it's in the speed band nothing happens no messages, however if it's approaching a speed limit waypoint and it's carrying excess decent speed it's smart enough to say look I'm in path but I'm going to bust the approaching speed limit as I was supposed to be doing 300kts but I've had to do 324kts due to unforecast winds so help me out and give me some drag so I can slow to the 280kts required at the next waypoint"

When it decides to pop this message is I don't know.

[cf6-80c2b5f]

Are you referring to the -8 with regard to the "speed band" or do you think the -400 also thinks that far ahead?

That is an interesting observation you make and would explain why probably every time I have seen DRAG REQUIRED, I have been doing less than MMO/VMO - 16.

It sure would be nice to see something in writing on this, but I can't seem to find much. The FMC User's guide is no help here.

[JammedStab]

I think you will find that there is more than one reason for a drag required message.

According to the Honeywell FMC guide for the 747, "The DRAG REQUIRED and THRUST REQUIRED messages are displayed to advise the pilot of speed changes required to maintain the descent path.". They don't give a lot more info about it.

I had one yesterday on a different Boeing but it was not because of airframe overspeed concerns. It was only to meet a speed restriction on a transition route at about 4000 feet. So if nothing had been done, I don't think it would have gone to VNAV SPD, it just would not have met the speed restriction without speedbrakes.

[cf6-80c2b5f]

The Boeing information on this is incredibly inadequate. It says nothing about displaying DRAG REQUIRED if you are on path but simply too fast. I think you are right -- speed is another factor that would set off the DRAG REQUIRED message.

[burgerbun]

Speed is the only factor that will give you a drag required message. And it is based on the speed entered in the FMS descent page or 3/3 as you said. Do you fly PS X?

[Avinthenews]

The 744 and -8 have exactly the same speed band, the only difference is it's actually on display on the -8 PFD speed tape, like the A330/340 etc have had for years.

As others have added "drag required" can occur for a couple of reasons one being the -16kts VMO condition the other being an approaching speed constraint that the jet is too hot to satisfy.

[cf6-80c2b5f]

Speed is the only factor that will give you a drag required message. And it is based on the speed entered in the FMS descent page or 3/3 as you said.

So, why the discrepancy between the Boeing manual (DRAG REQUIRED message at VMO-16/349 knots) and the statement in the Big Boeing Book that being more than 15 knots above FMC target descent speed will set off a DRAG REQUIRED? Which one is right? If FMC target descent speed was 280 and it took 300 to stay on path, would I get the message?

[AtoBsafely]

"If FMC target descent speed was 280 and it took 300 to stay on path, would I get the message?"

No, you wouldn't get the message until approaching Vmo. However, in years of operating the aircraft, I've never gotten much outside of the programmed speed +/- 20kt. At target speed -10 I adjust thrust to stay in HOLD, and if the speed is uncomfortably high, I start to use a little speedbrake (often there is another constraint ahead). It really isn't an issue.

[cf6-80c2b5f]

No, you wouldn't get the message until approaching Vmo.

Thanks for the input. I can remember seeing DRAG REQUIRED at times where I was nowhere near VMO-16. Probably most of us have seen the DRAG REQUIRED message when we were nowhere near 349 knots.

The FCOM I have only mentions that the DRAG REQUIRED message will appear at 349 knots (VMO/MMO-16) on a VNAV descent while attempting to stay in VNAV PTH. Nowhere else in the FCOM is it discussed. https://drive.google.com/file/d/0B-W...ew?usp=sharing

There obviously must be more than this one situation (349 knots) that would set off the DRAG REQUIRED message, but I can't find it in any material in my possession. It seems that there are a number of differeing opinions as to when you would see this message. I'm just trying to see if there's anything other than the above section that would shed some light on this.

[AtoBsafely]

Usually, you will see "DRAG REQUIRED" as you approach a speed+altitude constraint. Usually the FMC default 240/10000. At this point, VNAV says it needs drag to achieve the approaching constraint.

[cf6-80c2b5f]

Usually, you will see "DRAG REQUIRED" as you approach a speed+altitude constraint. Usually the FMC default 240/10000. At this point, VNAV says it needs drag to achieve the approaching constraint.

I wholeheartedly agree; but where is that written? I can't find it.

[JammedStab]

Originally Posted by cf6-80c2b5f

I wholeheartedly agree; but where is that written? I can't find it.

Somewhere hidden away in a storage room at Honeywell, perhaps. Apparently there are all kinds of VNAV formulas that just about all pilots are unaware of.

[Long Haul]

Hello,

My company's FCOM states that prior to reaching the first speed constraint on descent, the aircraft will descend in VNAV PTH and pitch down to as high as 354 knots (VMO/MMO - 11) to stay on path (at 349 knots the scratchpad will display DRAG REQUIRED). When even 354 knots won't keep it within 150' of the path, speed reversion occurs, the pitch mode goes from VNAV PTH to VNAV SPD, VNAV resets the target speed to 349 knots, and the scratchpad message DRAG REQUIRED displays again.

This seems like a really high speed to be flying to maintain the path and to give warning that drag is required. Was there a revision to the software at some point that allowed this increase, or am I reading this wrong? The Big Boeing Book mentions it will only go up to the FMC target speed + 15 in this situation and the DRAG REQUIRED will illimunate at that point. But it doesn't state whether this applies to the B744. Thoughts?

Thanks!

On the first page of the the portion of the FCOM you posted, system behaviour is described when the aircraft is above the first speed constraint (it will allow the speed to increase up to VMO -11 to maintain path), and on the second page it describes system behaviour below the first speed constraint ("the airplane may accelerate to 15 kts above target speed to maintain it's path). So, both are correct, depending on where you are on the descent. Once you enter a manual speed on the VNAV descent page, like .85/300, you have just entered the first speed constraint and the DRAG REQUIRED message will now be displayed once you reach 310 KIAS. So yes, you have seen the DRAG REQUIRED message at lower speeds than VMO -11. And yes, it is a very high speed for the FMC to try and maintain it's path at, so when I am starting a descent from FL370 or higher I will usually use speed intervention just before descending to bring the speed back a little to avoid it pitching down into an overspeed.

[cf6-80c2b5f]

Thanks for the reply.

Once you enter a manual speed on the VNAV descent page, like .85/300, you have just entered the first speed constraint and the DRAG REQUIRED message will now be displayed once you reach 310 KIAS.

This is an interesting theory and would certainly explain the DRAG REQUIRED displaying well prior to 349 knots. But why wouldn't it also apply to prevent the speed from going up to 354 knots. Once below the first speed constraint (as you said entering .85/300 would put you in that position on descent), the maximum speed to maintain the path would then be 15 knots above FMC target speed (page 2). In this case, if you entered .85/300, wouldn't it only go as fast as 315 knots to maintain the path before speed reversion occurs? Would there even be a need to speed intervene at Tod as you mention? I see that the second scenario on page 2 (after you have passed the first speed constraint) doesn't include a maximum mach. Are you suggesting that 315 knots would overspeed the mach?

If that's the case, it would be one of the rare cases where VNAV would not provide mach overspeed protection.

[aeo]

Hi mate, I think its important to remember the FMC calculates its TOD prediction point based on lots of inputs, ie;

1) Cruise altitude
2) Cruise speed
3) E/D point
4) 'And where applicable, waypoint altitude and speed constraints

Based on these numbers, the aircraft should descend at the economy cruise speed (AKA VNAV Cruise Descent) based on your cost index number with T/L's in Idle/Hold (albeit with a little help using THR or SPD BRKS if your abv or blw path)..

Therefore, under normal circumstances, and winds permitting, the aircraft should never get even close to those VMO/MMO -16 (-11) speeds quoted in the FCOM.

Cheers.