VNAV Flap Retraction / Acceleration Climb Gradient

[VHOJT 172]

Hi all,

I've been having a look through the manuals and I was wondering if anyone more enlightened than me might be able to answer this question

Does VNAV during flap retraction/acceleration provide any particular climb gradient, such as the standard 3.3%?  Or does it just try and provide a certain level of acceleration or perhaps a particular V/S?

Any insight is most welcome!

Cheers,

[downscc 1,327]

The pitch mode is pretty much a speed function during climb including flap retraction, if you are heavy enough there may be a period of time after thrust reduction that you only see 200-300 fpm climb.

I've pretty much abandoned VNAV in favor of hand flying the departure using the FD in TOGA mode and then initial climb I punch the FLCH button and bug up the speed to clean flaps up mark.  Once I've cleaned her up I'll put her in VNAV mode for remainder of climb.  I also use CLB power and then will select CLB1 or CLB2 as appropriate instead of using the derated climb for flap retraction segment of climb.  This really helps with those hot and heavy departures, and is a close approximation of what I've read B744 operators normally do.

Edited March 5, 2018 by downscc

[Mike777 163]

On 3/5/2018 at 11:28 AM, downscc said:

I've pretty much abandoned VNAV in favor of hand flying the departure using the FD in TOGA mode and then initial climb I punch the FLCH button and bug up the speed to clean flaps up mark.  Once I've cleaned her up I'll put her in VNAV mode for remainder of climb.  I also use CLB power and then will select CLB1 or CLB2 as appropriate instead of using the derated climb for flap retraction segment of climb.  This really helps with those hot and heavy departures, and is a close approximation of what I've read B744 operators normally do.

Would this also be common in a heavy 777?

Thanks,

Mike

[berts 97]

On 3/5/2018 at 6:34 AM, VHOJT said:

Does VNAV during flap retraction/acceleration provide any particular climb gradient, such as the standard 3.3%?  Or does it just try and provide a certain level of acceleration or perhaps a particular V/S?

That's a very good question! 

The simple answer is, no it doesn't.  If you think about it, there are many variables which affect the climb gradient, not least being the takeoff weight.  The climb gradient will therefore vary for every departure depending on the procedure and data fed into the FMC, but it should never be less than the minimum performance required for adequate terrain clearance.  At 400ft with the A/P and VNAV engaged, the speed is maintained at V2+10 to 25Kts up to the acceleration altitude.   At the thrust reduction height or Flaps 5 the FMC will adjust the VNAV path so that the aircraft accelerates until the greater of 250kts or VRef+100kts is reached.  This speed will then be maintained until it is cancelled by the pilot or removed automatically by the FMC as part of the normal speed transition for the departure airfield (e.g. 10,000ft).

Contrary to what Dan says - sorry Dan :-{  - VNAV is actually the preferred mode for virtually all takeoff, climb out and flap retraction procedures and you should normally use it whenever flying the QOTS with the F/D on or A/P engaged immediately after takeoff.  You should also select reduced thrust/derated climb whenever the performance data allows, because doing so will give you a better idea of the real aircraft's climb performance at all weights.  If VNAV is not used then - as Dan says :-)  - it is OK to use FLCH, but you will need to remember to set climb thrust using the THR switch and also set the commanded speed to the Flaps UP manoeuvering speed during flap retraction. In the event of an engine failure the climb gradient during Flap retraction will be almost level (i.e. 0 to 500'/min) with VNAV or FLCH engaged.  Incidentally, V/S is not used during takeoff and flap retraction because it does not provide speed limit protection.

Bertie Goddard

 

 

[Copper. 78]

I guess it depends on the operator, Bertie. Agree with all you’ve said except not using v/s during flap retraction and engaging a/p really early. I’ve seen the opposite.

Dans procedure for takeoff is more for 737NG than the widebodys and up, I suppose.

The 777 would use VNAV/LNAV in the same way Bertie said, normally.

Edited March 7, 2018 by Copper.

[downscc 1,327]

I always use VNAV for 737/777 departures, my modification to the 744 procedure was based on two sources I had on BAW procedures.  Agree, there needs to be a pilot involved to take care of flaps and bug speed but it doesn't seem incorrect, but of course no one should be using V/S mode in this regime of flight. Lordy no.

[skelsey 765]

3 hours ago, downscc said:

I always use VNAV for 737/777 departures, my modification to the 744 procedure was based on two sources I had on BAW procedures.  Agree, there needs to be a pilot involved to take care of flaps and bug speed but it doesn't seem incorrect, but of course no one should be using V/S mode in this regime of flight. Lordy no.

Dan,

I wouldn't want to doubt your source for a moment, but whilst FLCH used to be the mode of choice for go arounds, VNAV has been the recommended pitch mode since at least 2015; and I am almost 100% certain our friendly Simfest BA B744 TC would give us very weird looks (only exceeded by the weirdness of look we'd get for using V/S!) if we were to use FLCH in the inital climb after takeoff! Indeed, LNAV/VNAV armed is an item on the Preflight checklist, and there is no reference to use of FLCH during takeoff in any of the BA-specific manuals I have access to.

Not to say it can't be done, but I would hesitate to call it standard...

Also, for reference re: packs:

Quote

SP.2 Air Systems

Packs Off Takeoff
It is current BA policy to conduct Packs Off Takeoffs if TOW is greater than 300,000 kg, unless the Captain considers cabin environmental conditions are such that this is not appropriate.

Current as of 2017; it is possible there has been a change since then though...?

[downscc 1,327]

Super, thanks for the feedback Bertie and Simon.  Much appreciated.  My knowledge is limited and always growing, I hope.

[berts 97]

18 hours ago, Copper. said:

I guess it depends on the operator, Bertie. Agree with all you’ve said except not using v/s during flap retraction and engaging a/p really early. I’ve seen the opposite.

I agree with your comment about the operator and, as Simon has apparently pointed out, procedures will also change with the same operator too.  It's up to the handling pilot to decide when they engage an autopilot and in busy airspace it is probably more sensible to do this fairly soon after take-off.   However, I still maintain that V/S should not normally be used due to the lack of speed protection.

3 hours ago, skelsey said:

It is current BA policy to conduct Packs Off Takeoffs if TOW is greater than 300,000 kg, unless the Captain considers cabin environmental conditions are such that this is not appropriate.

Simon, this is interesting because above 300,000kgs TOW the APU to PACK procedure would normally be used with derated power if possible.  The PACKS OFF procedure is (or was?) used in order to gain an extra 1,900kgs in Takeoff Performance Limited (TOPL) weight over and above the Standard 524 performance tables and it would also be used if the APU is unserviceable AND the actual TOW is within 15,000kgs of the calculated PACKS ON TOPL weight.  Below 300,000kgs the TOPL data would normally be calculated using all A/C PACKS ON.

Although some airfields (e.g. Bogota) may have their own STD PACKS OFF takeoff data, unless the Takeoff performance is particularly limiting isn't it better to have at least one A/C Pack on for take off in order to maintain a suitable cabin environment for the passengers and crew?

Bertie  

[skelsey 765]

Hello Bertie,

12 minutes ago, berts said:

Simon, this is interesting because above 300,000kgs TOW the APU to PACK procedure would normally be used with derated power if possible.  The PACKS OFF procedure is (or was?) used in order to gain an extra 1,900kgs in Takeoff Performance Limited (TOPL) weight over and above the Standard 524 performance tables and it would also be used if the APU is unserviceable AND the actual TOW is within 15,000kgs of the calculated PACKS ON TOPL weight.  Below 300,000kgs the TOPL data would normally be calculated using all A/C PACKS ON.

Although some airfields (e.g. Bogota) may have their own STD PACKS OFF takeoff data, unless the Takeoff performance is particularly limiting isn't it better to have at least one A/C Pack on for take off in order to maintain a suitable cabin environment for the passengers and crew?

I agree in principle. I can only imagine that the question of APU fuel burn may possibly come in to the equation (on the principle of a small amount per flight x many flights per day = a large number) and that in temperate climates the short period of time for which the packs are off is liable to result in only a negligible change in comfort level? An acquaintance who fairly regularly flies ex Singapore has in the past confirmed that it does get rather toasty down the back during takeoff when it is +35C outside though!

The procedure itself calls for packs OFF as the very last line-up item prior to setting thrust, and then:

Quote

Prior to 3000 ft AAL and when climb
thrust is set:
• Set one Pack Control Selector
to NORM, allow cabin rate to
stabilise.
• Set the remaining Pack
Control Selectors to NORM.

 Both the "Configure air conditioning packs as needed" item in the line-up flow and the above quote are BA additions rather than Boeing original text; as I say, current as of 2017 (which is post the "Back to Boeing" push which saw an overhaul of the procedures back toward Boeing standard) but it is possible it may have changed in the meantime; it is certainly the procedure I have always been familiar with, however. (Incidentally -- the Performance Manual also states "When the actual take-off weight is 300,000 kg or below a PACKS ON take-off should be carried out. When actual take-off weight is over 300,000 kg a PACKS OFF take-off should be carried out.")

CARD to my knowledge (and, again, it is possible it may have changed in the last year or so) provides only figures for full TO thrust (with an assumed temperature de-rate) as standard. TO1 or TO2 rating may be selected by entering a correction code as part of the ACARS request; of note also (now that I check the books) I note that there is also a correction code for "APU Inlet door inop in open position" which attracts a TOPL penalty of 1,452kg. As CARD optimises for the highest possible TOPL, I wonder if this could be a factor as well (I presume that as the inlet door will be open with the APU running the same penalty must apply?!).

[jon b 2,047]

On 05/03/2018 at 6:34 AM, VHOJT said:

Hi all,

I've been having a look through the manuals and I was wondering if anyone more enlightened than me might be able to answer this question

Does VNAV during flap retraction/acceleration provide any particular climb gradient, such as the standard 3.3%?  Or does it just try and provide a certain level of acceleration or perhaps a particular V/S?

Any insight is most welcome!

Cheers,

In a normal climb out VNAV uses 40% of thrust for acceleration and 60% for climbing.

During an engine out climb it uses the reverse priority , 60% for acceleration and 40% for climb.

Academic discussion point only really as you'll not notice any difference. The aircraft is certified to have at least 1.5 % climb gradient with an engine out.

So I guess a minimum of 1.5% climb gradient would be your answer 

Cheers

Jon Bunting

[berts 97]

On 3/7/2018 at 11:58 PM, skelsey said:

(Incidentally -- the Performance Manual also states "When the actual take-off weight is 300,000 kg or below a PACKS ON take-off should be carried out. When actual take-off weight is over 300,000 kg a PACKS OFF take-off should be carried out.")

CARD to my knowledge (and, again, it is possible it may have changed in the last year or so) provides only figures for full TO thrust (with an assumed temperature de-rate) as standard. TO1 or TO2 rating may be selected by entering a correction code as part of the ACARS request; of note also (now that I check the books) I note that there is also a correction code for "APU Inlet door inop in open position" which attracts a TOPL penalty of 1,452kg. As CARD optimises for the highest possible TOPL, I wonder if this could be a factor as well (I presume that as the inlet door will be open with the APU running the same penalty must apply?!).

Thanks for the % figures Jon, although I am 100% certain you will notice a BIG difference in climb and acceleration performance between a lightweight and a heavyweight take-off on 4 or 3 engines!

I suppose having only two procedures in CARD (Packs on or off) as opposed to three reduces the risk of a mistake being made.  The 75 code for use in CARD with the APU Inlet Door being open does not give a correction figure in the notes I have. However, applying a 1,452 kgs penalty for the drag of the APU door being open seems like overkill to me.  As far as APU to Pack performance manual corrections is concerned, a 100kgs correction is (was?) applicable if the performance was Field Length limited and 400kgs if it is (was?) Climb (WAT) limited.

The TO1 and TO2 derates are intended for use on contaminated and VMCG limited runways so they are not normally included in any of the normal take-off summaries.  TO2 is intended for use on three engine ferries.

Bertie   

[VHOJT 172]

Gentlemen, as always, a very interesting discussion!  The knowledge in this forum is always impressive.  Thanks for your replies! 

Say you have a SID like those at OMDB which wants a 5% gradient up to 8000', with a lot of tight altitude at or above restrictions along the way.  Now usually SIDs like this on the Jepps give a table for G/S vs V/S, or you could do a little bit of maths to find the required V/S.  According to the chart, at 250 knots, that is 1250fpm, at 300 knots, that is 1500fpm.  Does an airline performance person/department look at this, and work out a flap retraction/thrust reduction profile to suit?  Because as far as I can tell, there is really no way for a pilot to know exactly what average gradient they are achieving?  In addition, there is no way a heavy 744 is going to make those V/S requirements when accelerating, so you would need to be "above the average gradient" before accelerating, when your required V/S would be less than the gradient requirement.  I can't imagine holding off until 8000' for acceleration?

The only way I've managed in the sim to meet the at or above requirements out of OMDB with a full 744 is:  T/O thrust until 4000'.  At 1000', VNAV speed intervene and bug up to flap 10 retraction speed, then climb with T/O thrust and flap 10 until 4000'.  All the while I've really got no idea what gradient I've been averaging.  I am guessing meeting the at or above requirements would mean satisfying the gradient.

I've got myself in a muddle writing that.  Head is about to explode.
 

 

 

[berts 97]

On 3/10/2018 at 10:18 PM, VHOJT said:

Say you have a SID like those at OMDB which wants a 5% gradient up to 8000', with a lot of tight altitude at or above restrictions along the way.  Now usually SIDs like this on the Jepps give a table for G/S vs V/S, or you could do a little bit of maths to find the required V/S.  According to the chart, at 250 knots, that is 1250fpm, at 300 knots, that is 1500fpm.  Does an airline performance person/department look at this, and work out a flap retraction/thrust reduction profile to suit?  Because as far as I can tell, there is really no way for a pilot to know exactly what average gradient they are achieving?  In addition, there is no way a heavy 744 is going to make those V/S requirements when accelerating, so you would need to be "above the average gradient" before accelerating, when your required V/S would be less than the gradient requirement.  I can't imagine holding off until 8000' for acceleration?

Performance criteria for all of the airfields the B744 operates from should be provided to Flight Crew by their airline by means of an onboard Performance Manual (i.e. in Booklet or iPad/Tablet format or similar), so they shouldn't need to work out any climb gradients or rely on tables published in Jeppesen charts etc. 

On the day, provided any Performance Restrictions for that runway are followed and nothing major happens which affects the aircraft's performance then there should never be an issue in complying with the approved SID (or STAR).  All Engines Operative and Engine Failure procedures are normally shown separately for a SID which cannot be flown using standard clean up/operating procedures and there will usually be a detailed Performance Restriction note added to that particular instrument departure.  For example, the Performance note on the SID might say 'maintain V2 +10kts to xxxxx Acceleration Altitude' (i.e. a non-standard height before cleaning up); or 'maintain runway heading to a xxx DME' before turning onto a specified track.  In some cases it might be possible to comply with only one SID out of several possibilities when departing an airfield in a certain direction, in which case the Performance Restriction note might simply say 'xxx SID - No restrictions. All Others Prohibited'.

Let's say ATC imposes an additional tight altitude restriction during the initial climb.  The Flight Crew will have several options available to them in order to comply without necessarily having to delay Flap retraction.  These could include the use of speed intervene, selecting max angle and/or Climb Thrust in the FMC and the use of the Altitude Range Arc which is based on vertical speed and ground speed predictions.  There is always the option of saying to ATC " Unable to Comply" if you think you are not going to be able to make a certain altitude, speed restriction or ETA.

Bertie

[jon b 2,047]

On ‎10‎/‎03‎/‎2018 at 10:18 PM, VHOJT said:

 

Say you have a SID like those at OMDB which wants a 5% gradient up to 8000', with a lot of tight altitude at or above restrictions along the way.  Now usually SIDs like this on the Jepps give a table for G/S vs V/S, or you could do a little bit of maths to find the required V/S.  According to the chart, at 250 knots, that is 1250fpm, at 300 knots, that is 1500fpm.  Does an airline performance person/department look at this, and work out a flap retraction/thrust reduction profile to suit?  Because as far as I can tell, there is really no way for a pilot to know exactly what average gradient they are achieving?  In addition, there is no way a heavy 744 is going to make those V/S requirements when accelerating, so you would need to be "above the average gradient" before accelerating, when your required V/S would be less than the gradient requirement.  I can't imagine holding off until 8000' for acceleration?

The only way I've managed in the sim to meet the at or above requirements out of OMDB with a full 744 is:  T/O thrust until 4000'.  At 1000', VNAV speed intervene and bug up to flap 10 retraction speed, then climb with T/O thrust and flap 10 until 4000'.  All the while I've really got no idea what gradient I've been averaging.  I am guessing meeting the at or above requirements would mean satisfying the gradient.

I've got myself in a muddle writing that.  Head is about to explode.
 

 

 

If the departure is coded in the FMC with the crossing restrictions and the climb is looking marginal then the FMC will alert you with an " UNABLE NEXT ALT" message in the scratchpad. You then have 3 options available, as you've mentioned, speed intervene ,and or remove any climb power derate , or thirdly simply advise ATC you are unable to meet the vertical profile of the SID. The restrictions are most likely airspace related and so it's ATC's problem if on the day you cant quite make it despite your best efforts. If however the restrictions were related to terrain or obstacles this would have been considered in your take off performance data and would be reflected in being issued with a reduced RTOW which would ensure you met the required gradients. As Bertie mentions, on such departures where terrain or obstacles are an issue and the RTOW is reduced  there is often a performance supplemental procedure commonly called an " emergency turn" procedure to follow in the event of an engine failure or performance issue ,such as the gear not retracting, which is required to be flown to validate the RTOW.

 

Cheers

Jon Bunting