Was doing a flight from EGPF - EGKK and using the NATS SRD. PFPX said i would land with 2.1 tonnes and my reserves are 2.0 tonnes at Gatwick. But soon as i modify the STAR (enter HARD altitudes at DISIT FL200 and KIDLI FL150) the fuel on the progress page drops to 2.0 and sometimes 1.9 causing the FMC to show USING RSV FUEL.

 

Can someone explaing why the fuel burn increase? I thought the FMS would recalculate an idle DES path atleast.

I think you are looking for more precision than either the FMS or aircraft are capable of delivering, and this would be true in the actual aircraft as well as the sim.

 

You are leaving too small a margin between the expected landing fuel, and your reserve amount. If the FMS originally predicted 2.1 tonnes on landing, and you entered a reserve of 2.0, you are leaving yourself a margin of only 100 kilos before you start using reserve fuel. The slightest variation in actual conditions in upper winds or temps vs. what was predicted - an ATC-mandated hold - a slight re-routing - anything - could use that 100 kilos in a flash.

 

You probably should carry more fuel to start with, or use a smaller reserve amount.

 

DISIT is 24 nm from KIDLI, and you need to lose 5000 feet between them. With hard altitude targets at both, the FMS is probably going to have to reduce your rate of descent at DISIT to avoid getting to 15,000 too soon before KIDLI. If you had been using an idle power descent, reducing your descent rate is going to require that the aircraft add power passing DISIT, to avoid having your speed drop, which would account for the reduction in your predicted landing fuel.

 

The FMS can only do so much with an idle descent. At idle, your speed is controlled by pitch, and if the spacing between two hard altitude constraints is such that the aircraft has to reduce pitch to hit the second constraint at the right altitude, it is going to have to add power to hold your speed.

 

Is DISIT part of the STAR? My Navigraph charts for EGKK show it beginning at KIDLI, with an "expect 15000" note. If you had entered a a 15000 constraint ONLY at KIDLI, but not the 20000 constraint at DISIT, you might have been better off - but perhaps my chart is missing a segment.

just to clarify is this the route you are using? it's what i get from the SRD.

 

NORBO1J NORBO UT256 DCS UN864 SUBUK DCT KEPAD UL151 TELBA L151 KIDLI WILLO3B

 

what fuel setting are you using? with EU-OPS i get 2.4 reserve and 5.1 at landing listed (the difference mainly being the alternate fuel for egll) with a FL330 cruise.

 

it seems like that is a very small buffer as if you were planning without reserve, and alternate fuel only?

 

anyway without loading and flying it i can only speculate at the change but usually a hard restriction is going to be suboptimal compared to the idle descent, maybe the distance between DISIT and KIDLI is too long that there is no way to fly it at idle and still hit both restrictions exactly, like at idle and econ des airspeed you will reach 15000 before the waypoint and have to burn some extra while cruising at 15000. or, a perfect descent angle would be too slow without using more than idle thrust....maybe...

EDIT: oops, JRBarret answered this part more precisely while i was typing it up

 

cheers

-andy crosby

Is DISIT part of the STAR? My Navigraph charts for EGKK show it beginning at KIDLI, with an "expect 15000" note. If you had entered a a 15000 constraint ONLY at KIDLI, but not the 20000 constraint at DISIT, you might have been better off - but perhaps my chart is missing a segment.

 

DISIT is part of the UL151 and L151 airways that are part of the SRD route.. you can see it as an inset in the WILLO STARS chart, as part of the holding point at DELBO. maybe that restriction would only be assigned if you were assigned the hold? gatwick looks complicated

 

cheers

-andy crosby

And to add to what Andy said, I think you may misunderstand what reserve fuel is. "Reserve" fuel in the FMS is NOT the amount that you expect to have on landing. That is "trip fuel". Under ICAO, you should also carry (at a minimum) 5 percent of your predicted trip fuel for unexpected re-routings or delays (contingency fuel) PLUS sufficient fuel to fly for 45 minutes at normal holding speed at an altitude of 1,500 above the destination airport elevation - and that is with no alternate airport. WITH an alternate, your must also include sufficient fuel to fly one missed approach, climb to the required cruise altitude to reach your alternate, fly to the alternate, and conduct an approach and landing.

 

The required reserve fuel varies between the U.S. and Europe from a regulatory standpoint, but it definitely not the amount you expect to have on landing at your destination - it is always going to be more. The long and short of it is that you needed more fuel to start with.

 

DISIT is part of the UL151 and L151 airways that are part of the SRD route.. you can see it as an inset in the WILLO STARS chart, as part of the holding point at DELBO. maybe that restriction would only be assigned if you were assigned the hold? gatwick looks complicated

Ah! I missed the inset on the chart at first glance. Thank you!

I guess maybe the impression in the sim world is that flights are flown exactly as planned. I understand how this could be the case, with all the emphasis put on the nitpicky details in planning. The thing to understand is that planning requirements are basically just to be legal to dispatch. Then there's real life. You are never ever going to calculate your fuel required to dispatch legally, put exactly that much fuel on, and go. That leaves no room at all for the million little things that don't go according to plan during a flight. The published altitudes on a STAR are a good starting point but they're often modified by ATC. Delay vectors and holds are a reality for traffic. Heck, being asked to slow up early, requiring dirtying up early, will burn a few hundred pounds (or kilos).

 

Basically, it's not worth trying to manage fuel to a couple hundred kilos.

I think you are looking for more precision than either the FMS or aircraft are capable of delivering, and this would be true in the actual aircraft as well as the sim.

 

You are leaving too small a margin between the expected landing fuel, and your reserve amount. If the FMS originally predicted 2.1 tonnes on landing, and you entered a reserve of 2.0, you are leaving yourself a margin of only 100 kilos before you start using reserve fuel. The slightest variation in actual conditions in upper winds or temps vs. what was predicted - an ATC-mandated hold - a slight re-routing - anything - could use that 100 kilos in a flash.

 

You probably should carry more fuel to start with, or use a smaller reserve amount.

 

DISIT is 24 nm from KIDLI, and you need to lose 5000 feet between them. With hard altitude targets at both, the FMS is probably going to have to reduce your rate of descent at DISIT to avoid getting to 15,000 too soon before KIDLI. If you had been using an idle power descent, reducing your descent rate is going to require that the aircraft add power passing DISIT, to avoid having your speed drop, which would account for the reduction in your predicted landing fuel.

 

The FMS can only do so much with an idle descent. At idle, your speed is controlled by pitch, and if the spacing between two hard altitude constraints is such that the aircraft has to reduce pitch to hit the second constraint at the right altitude, it is going to have to add power to hold your speed.

 

Is DISIT part of the STAR? My Navigraph charts for EGKK show it beginning at KIDLI, with an "expect 15000" note. If you had entered a a 15000 constraint ONLY at KIDLI, but not the 20000 constraint at DISIT, you might have been better off - but perhaps my chart is missing a segment.

 

The reason for the margin was, i wanted to see how accurate PFPX is running with minimum fuel with no extra. I do however carry an extra 300kg but on this flight i left it out. PFPX is very accurate as we know. You will find DISIT and KIDLI descent planning on AOI 4 of Navigraph. I however done the flight again without the restrictions and the acft descent with IDLE power and arrived at EGKK with 2.1

 

Is there anyway i can force PFPX to calculate these restrictions at the planning stage? In the real world would this all be done my flight planners?

just to clarify is this the route you are using? it's what i get from the SRD.

 

NORBO1J NORBO UT256 DCS UN864 SUBUK DCT KEPAD UL151 TELBA L151 KIDLI WILLO3B

 

what fuel setting are you using? with EU-OPS i get 2.4 reserve and 5.1 at landing listed (the difference mainly being the alternate fuel for egll) with a FL330 cruise.

 

it seems like that is a very small buffer as if you were planning without reserve, and alternate fuel only?

 

anyway without loading and flying it i can only speculate at the change but usually a hard restriction is going to be suboptimal compared to the idle descent, maybe the distance between DISIT and KIDLI is too long that there is no way to fly it at idle and still hit both restrictions exactly, like at idle and econ des airspeed you will reach 15000 before the waypoint and have to burn some extra while cruising at 15000. or, a perfect descent angle would be too slow without using more than idle thrust....maybe...

EDIT: oops, JRBarret answered this part more precisely while i was typing it up

 

cheers

-andy crosby

 

DISIT is part of the UL151 and L151 airways that are part of the SRD route.. you can see it as an inset in the WILLO STARS chart, as part of the holding point at DELBO. maybe that restriction would only be assigned if you were assigned the hold? gatwick looks complicated

 

cheers

-andy crosby

 

My route NORB1J NORBO UT256 DCS UN864 SUBUK DCT KEPAD UL151 TELBA L151 KIDLI WILO3B so exactly the same.

 

I'm using EU OPS. thats strange i'm getting different figures from you.

And to add to what Andy said, I think you may misunderstand what reserve fuel is. "Reserve" fuel in the FMS is NOT the amount that you expect to have on landing. That is "trip fuel". Under ICAO, you should also carry (at a minimum) 5 percent of your predicted trip fuel for unexpected re-routings or delays (contingency fuel) PLUS sufficient fuel to fly for 45 minutes at normal holding speed at an altitude of 1,500 above the destination airport elevation - and that is with no alternate airport. WITH an alternate, your must also include sufficient fuel to fly one missed approach, climb to the required cruise altitude to reach your alternate, fly to the alternate, and conduct an approach and landing.

 

The required reserve fuel varies between the U.S. and Europe from a regulatory standpoint, but it definitely not the amount you expect to have on landing at your destination - it is always going to be more. The long and short of it is that you needed more fuel to start with.

 

Ah! I missed the inset on the chart at first glance. Thank you!

 

I know what reserve fuel is. its ALT plus HOLD fuel at 1500 above the airport for 30 minutes you add both figures which give you BINGO fuel. Its not 45 minutes its 30 mins. well over in EUROPE anyway.

 

I don't think i needed more fuel in the way PFPX calculated it. It was my own fault going off what was planned and entering 2 hard alt rest. 

 

The sum of my reserve fuel was HOLD FUEL plus ALTERNATE FUEL was 2.0 tonnes so i entered 2.0 in the PERF INIT page and PFPX predicted i would land with 2.1 (very small margin) but i just wanted to see how accurate PFPX was. 

 

 

 

          PLANNED FUEL

----------------------------------

FUEL            ARPT   FUEL   TIME

----------------------------------

TRIP             LGW   2808   0106

CONT 5%                 169   0005

ALTN             STN    928   0019

FINRES                  998   0030

ADD RES                   0   0000

----------------------------------

PLANNED T/OFF FUEL     4903   0200

----------------------------------

T/OFF FUEL             4903   0200

TAXI             GLA    150   0015

----------------------------------

BLOCK FUEL       GLA   5053   0215

PIC EXTRA             .....

TOTAL FUEL            .....

REASON FOR PIC EXTRA  ............

--------------------------------------------------------------------

FMC INFO:

RESERVES               1926

TRIP+TAXI             .....

--------------------------------------------------------------------

 

 

 

Andy i'm curious why your getting different figures from me? My cruise alt was FL410 my alternate was EGSS. 

me?y cruise alt was FL410

Vernon

Why do you fly so high?

I ask the same thing lol its PFPX that does it

I know what reserve fuel is. its ALT plus HOLD fuel at 1500 above the airport for 30 minutes you add both figures which give you BINGO fuel. Its not 45 minutes its 30 mins. well over in EUROPE anyway.

 

BINGO Fuel is a mil term, and is only used there because it's the fuel point at which you must return to base. Since airline flights don't often return to base, the term is not relevant and should not be used. Some genius recently semi-famously used it when he declared an emergency so that he could land at a closed field being used for airshow practice: http://www.reviewjournal.com/news/las-vegas/allegiant-air-execs-controls-flight-landed-low-fuel

 

It's best not to perpetuate the practices of those dullards.

The reason for the margin was, i wanted to see how accurate PFPX is running with minimum fuel with no extra. I do however carry an extra 300kg but on this flight i left it out. PFPX is very accurate as we know. You will find DISIT and KIDLI descent planning on AOI 4 of Navigraph. I however done the flight again without the restrictions and the acft descent with IDLE power and arrived at EGKK with 2.1

 

Is there anyway i can force PFPX to calculate these restrictions at the planning stage? In the real world would this all be done my flight planners?

 

That's a good question. I assume that the altitude constraints at DISIT and KIDLI were not encoded into the FMS version of the STAR, since you had to enter the constraints manually - which is easy to do in the FMS.

 

Since PFPX uses the same data source, the constraints probably are not hard-coded in the planner either. I don't know if it is possible to enter altitude or speed constraints in a PFPX SID or STAR in the planning stage.

 

I think the reason the constraints are not pre-coded into the procedure is because of the word "expect" in the altitude at KIDLI. In other words, the constraint is assigned by ATC, and though one will probably be told to cross at 15,000 99% of the time, there may be occasions where a different altitude restriction is given by the controller.

So how can we best plan for these possible constraints from ATC? How much fuel do we have to carry extra for the increased thrust?

 

I'm still very curious why Andy Crosby got those higher figures? Lower cruise alt ?

So how can we best plan for these possible constraints from ATC? How much fuel do we have to carry extra for the increased thrust?

 

I'm still very curious why Andy Crosby got those higher figures? Lower cruise alt ?

 

if i recall correctly, there's an 'advanced' tab in pfpx that has a spot where you can add altitude restrictions to waypoints in the route..although i don't think it lets you add them to points contained inside the STAR, i'm pretty sure you could add the 15k restriction to KIDLI at that spot. since DISIT is a waypoint before the star it might be possible to add it there also. i'm at work and don't have pfpx installed here so i can't really be more specific without looking at it.

 

you can also set a max cruise level on the main page which lets you pick a more realistic value for the planning of short flights, without it pfpx wanted a FL390 for mine, which nearly puts the TOC and TOD right next to each other.  a friend of mine  works as an attendant for ryanair, i always bug her to ask the pilots for details, according to a first officer there, for the engx->eidw route they are anywhere from FL230 to FL320 but 'normally quite low' so i sort of use that as a guideline when planning shorter hops around the UK.

 

although the lower cruise alt would probably use a bit more fuel i think it's only a small percentage on a shorter hop like this. i think the reason my numbers were so different was that i was using lbs instead of kgs, at a quick glance it looks like your numbers are about half of mine which might explain that

 

cheers

-andy crosby

I'm still very curious why Andy Crosby got those higher figures? Lower cruise alt ?

Vernon

In addition to what Andy just wrote, maybe he is using a different cost index and in PFPX the 738 has a different fuel bias and drag.

Hi andy maybe when your home you could look into the advanced tab? I just tried it and it doesn't have an effect? Not sure how to work it.

 

I have a friend who i speak to that flys for Ryanair and i have there SOPs and loads of other things had a wee look through all the DOCS and nothing mentions short hops etc!

Michael thats right i use CI 6 per my Ryanair SOPs

What do you guys set the BIAS too?

My fuel burn bias in PFPX is left at 100% for all 777/737 airframes. I played with that setting for a couple of months and concluded the default value is the best for all around.

 

I use the advanced tab for planning when I am oceanic, it provides a way to force the step before entering the track. Not sure it's worth the extra minute it takes to set it up.  Keep things in perspective here, you are talking a 100 lb variance which is only 1% for a typical short-medium haul trip. Airframes will vary much more than that between tail numbers. This is an example of a simism being discussed elsewhere.