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PNR and trip fuel....

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MissSoph created the topic: PNR and trip fuel....

So I’m trying to get my head around the trip fuel when working out pnr...
say for an example... p123 of the performance book...
I hope I have this correct... but trip fuel is the amount of fuel req from take off to touch down...
So to find safe endurance... first find the trip fuel available... then convert the trip fuel to minutes at the cruise rate.... so I’m this question I have been given fuel on board... and I just do
Total fuel on board-fixed reserve-taxi -holding..... gives me 107 liters....
but why is it then, that if trip fuel is the fuel required to complete my take off to touch down, that when I calculated it as distance x fuel flow/gs out.... I end up with 80litres???
Question 11 p122 uses this method to calculate the PNR.... I’m just confused as to why one way... then another question you do it another.... is it to do with the fact that one question gives you the fuel on board? But why would it not equal the same.... it’s still both trip fuel?????
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John.Heddles replied the topic: PNR and trip fuel....

but trip fuel is the amount of fuel req from take off to touch down...

Refer CAAP 234-1(2.1) at www.casa.gov.au/sites/default/files/caap...or-aircraft-fuel.pdf .

Definitions on p8 gives the following story -

The amount of fuel required to enable the aircraft to fly until landing at the destination aerodrome, taking into account the operating conditions in paragraph 3.3. This includes (as applicable):

a. fuel for take-off and climb from departure aerodrome elevation to initial cruising level/altitude, taking into account the expected departure
routing, and

b. fuel for cruise from top of climb to top of descent, including any step climb/descent, and

c. fuel from top of descent to the point where the approach is initiated, taking into account the expected arrival procedure, and

d. fuel for conducting an approach and landing at the destination aerodrome.


So to find safe endurance... first find the trip fuel available... then convert the trip fuel to minutes at the cruise rate....

.. keeping in mind that this may need to account for things such as OEI and pressurisation failures if you are flying a fancy light twin. A better approach is to figure out a point from which you have a simple out and return to the PNR (whether that is at the same level, same IAS, same fuel flow or not), figure out all the other bits of fuel requirements and then run the PNR calculations on the bit left for playing with the PNR.

.... I end up with 80litres???

I don't have any idea of what the detailed question is - you need to post a copy of the question for discussion on specifics.

Question 11 p122 uses this method to calculate the PNR.... I’m just confused as to why one way... then another question you do it another....

Again, you need to specify the questions to get comments. Keep in mind that the text books get revised and another edition/revision might not have the same page details as your particular copy.

However, if you keep things simple, it is a straightforward exercise. In all cases, you have a simple table to fill in to figure out the fuel data. Fill this table out, every time, and you can't go wrong. Fill in the bits you know, then figure out the bits you don't know and then fill them in. Easy peasy.

Try not to have ten different "fuel planning" techniques in mind - it is all the same - you start with the basic table, fill in what you know and then set out to figure the bits you don't know - fill them in and Bob's your uncle.

You may come across different table layouts but they all do the same sort of thing and will be similar to the following -

trip fuel =
variable reserve = [may not be required and will vary according to category, etc]
alternate fuel = [may not be required]
traffic holding fuel = [may not be required]
weather holding fuel = [may not be required]
taxi fuel at departure = [there is no reason why you can't take final taxi out of the fixed reserve allowance UNLESS you are at an intermediate stop and not refueling]
fixed reserve =
total required fuel = [add it all up]
margin fuel = [ie spare fuel carried for whatever reason]
total usable fuel at start = [ie what you have usable in the tanks at start]

You might like to post the two questions which are causing you grief and we can have a play with them ....

Engineering specialist in aircraft performance and weight control.
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MissSoph replied the topic: PNR and trip fuel....

Thanks john... I’ll post the questions... but I think I may have my idea of what trip fuel is a bit skew whiff... so in the material.. trip fuel I was under the idea was the fuel burnt from point a to b.? But in fact... as you have written... trip fuel is the fuel burn plus all of the reserves and holding?

So the first question is as follows..... (it’s p122 of the study material)
Charter flight of a/c details...
TAs-180kt. Variable 10% trip fuel
Holding rate 30 litre/hr. Cruise 50 l/hr
Fixed reserves 45 mins at cruise. Taxi allowance 5litres
FPT 330m. Variation 10E. Distance a to b 244nm
Holding req at A 60 mins. GPTW wind 36030kt
Holding at B 30 mins
Fuel on board at start up 190 litres
Find the distance from A to PNR
To find the trip fuel for this problem, explanation is total fuel on board -fixed-taxi-holding at A /1.1


Yet another question....(p122 quest 11)
Data relates to a charter flight
Total distance 300nm
TAS cruise 150kt
Wind during cruise 15kt tailwind
Fuel flow during cruise 56 litres/hr
Fixed reserve 45 min at cruise rate
Start up 5 litres
Start up with min fuel required
Find distance to PNR.....
Explanation shows that the trip fuel formula was used...300x56/165

Now I’m just trying to understand why when both of the questions I have to solve PNR...so I have to calculate trip fuel, two different techniques have been used to calculate the trip fuel.... that’s what throws me with all of these questions.... seemingly simular questions, and all of the sudden the technique to answer changes.... this is where the whole performance subject has me stumped.... what key words, questions, ect... should I ask myself when reading the questions... is it down to the info they give me?
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John.Heddles replied the topic: PNR and trip fuel....

But in fact... as you have written... trip fuel is the fuel burn plus all of the reserves and holding?

Not at all - appears I may have confused the issue somewhere for you. Trip fuel (flight fuel, planned burn - whatever term you might prefer) is what you expect to burn, other things being equal. Reserves and other stuff is extra and nothing to do with trip fuel.

Let me give you a fuel planning motor car analogy - I drive a Nissan Xtrail which runs about 10 km/l typically - I also drive a 13 tonne truck for fun with quite different fuel consumption but let's figure it out on the little machine. Say I am intending to run from Sale in Victoria up to Canberra in the ACT. Distance via the Monaro is around 520 km, takes around 6 hours and, to a rough approximation, should burn around 52 litres. Does this mean that I could drain my tank, put in 52 litres and comfortably get to Canberra ? Probably not, once I have a think about things, especially the following points. If the trip were on good, level roads, without any traffic disruption, I would probably burn very, very close to 52 litres. However, running this road it's winding, goes up quite a climb from Cann River to the high country, and has a lot of heavy trucks to contend with. Probably, I will burn something in excess of 52 litres. However, say I don't have any useful data to figure out what the burn might likely be. So, I might start off the calculation by figuring a best guess as to a routine, anticipated burn off, about, 52 litres. This would be my (planned, or anticipated) trip fuel.

However, unless I am feeling pretty stupid today, I figure that I might need a bit more to allow for things which could increase the burn for this particular trip, so I reckon that I might add, say, 10% extra for those matters. That is I am going to add 5.2 litres as a reserve because I figure that might be reasonable for the run up to Canberra. Now, were I to be going somewhere else, I might figure a bit higher or lower for a suitable particular trip reserve. Overall, my historical driving in this car suggests that around 10 % extra will keep me safe from running out of fuel. due to routine route specific problems. I might call this reserve my variable reserve because I may or may not use some of it and I may use different proportions on different trips, depending on the circumstances. Either way, I figure that it's a good strategy to carry that sort of reserve pad to give me a good probability of not running out of fuel.

Now, there may be some other problems on the trip which I can't reasonably foresee - things like roadworks along the way where I might have any number of spots where I have to stop and will be idling for, say, 5-10 minutes while the folks on the road are doing their thing. It might be a good idea if I carry a bit extra fuel to allow for these unexpected things. I might call that extra fuel a traffic holding reserve. Looking at my history doing this trip over the past 20 years, I might allow, say, 30 minutes at idling fuel flow. Let's say that a figure of around 1-2 litres might do the trick.

There might be other problems as well. At the top of the run up the Cann Valley, we need to go over a river at Bombala. With a reasonable bit of rain, that floods and the highway is closed, kaput !. Say it's been raining and there is more rain forecast. I might like to carry some extra fuel to allow a significant diversion around the village and I might call this a weather holding reserve allowance. Now, this is a bit different to weather holding for an aircraft but the principle is similar in that, if you don't have it and (say for the car, there is no fuel available today at Bombala) you just might end up being embarrassed. Better to carry some extra fuel, just in case of what might be a reasonably foreseeable event so that I know I can get to somewhere I can fill up the tanks (believe me, I have been caught out at Bombala super big time coming south and had to spend the whole day heading back up through Canberra to the Hume, down to Albury way and then down to Bairnsdale - made a relatively easy quick drive an absolute marathon). So I might, for this situation at Bombala, allow, say, an extra 10 litres (around 100 km) for "weather holding".

Now, what other things can go wrong ? Maybe I have the traffic problems and the hill climb increased fuel burn problems and then get to Canberra and, being a bit tired today, take a wrong turn and head off towards Goulburn. Eventually, I realise my mistake, turn around, and head back down to Canberra but, in so doing, I am burning a fair bit of additional fuel - if I didn't have anything extra up my sleeve to allow for this sort of problem, then I am going to be bone dry and on the side of the road out in the paddocks north of Canberra and in a world of hurt. So I figure that it is a good idea always to have some extra fuel in the tank just in case something untoward crops up at the destination - I'd probably add, say, 45 minutes to an hour (75-100 km) to cover this sort of eventuality and I might call that my fixed reserve. I would be very careful to make sure that I don't inadvertently use any of that along the way up to Canberra as I have included it to cover me for last minute problems when I get to Canberra - no point having allowed for it at Sale and then burn it off on the way up the Monaro.

Sound a bit like fuel planning for your aeroplane ? Of course it is - all same same, just a different vehicle.

Let me have a looksee at the problems and see what I can come up with. If my answers are different to the book's I might need you to photograph the questions on your mobile and post the actual details so we can see what is what. Back a bit later on.

Engineering specialist in aircraft performance and weight control.
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John.Heddles replied the topic: PNR and trip fuel....

Back now with a few minutes to run the questions. Looks pretty straightforward. I get 187nm and 135nm. Were these the sorts of answers you were looking for ?

Your comments regarding the solution approaches appear to be reasonable so I am a bit confused as to where your specific problem lies.

so I have to calculate trip fuel

Correct

two different techniques have been used to calculate the trip fuel

Not at all, so far as I can see. Going back to my earlier comments - you start with the fuel calculation table, fill in what you know at the start of the question, figure out the other numbers, plug them in, do the additions and you have trip fuel read off from the table. Then just run the standard calculations. The only concern is that the 2*TAS thing starts to go off the rails for high drift angles but, for most problems, it's fine, certainly OK for these two.

that’s what throws me with all of these questions.... seemingly similar questions, and all of the sudden the technique to answer changes

Harken back to what I suggested previously. At this stage, don't try to develop a squillion different techniques for every slight variation in question style.

PNR is PNR - fill in the fuel table to get the trip fuel and then do the time and distance standard calculations for the PNR. About the only thing you need to take extra care with is to figure the plan out (in some cases, but not these) to a point down track from where you can run the stock standard equations for time and distance to the PNR.

Sometimes folks get a bit clever and take rule of thumb shortcuts. No problems with that, once you know what you are doing. After all, the experienced pilot spends much of his/her life running mental and back-of-a-fag-packet calculations day in, day out. However, when you are starting out, it probably is better to stick with the basic step A-step B-step C until you get the stuff comfortably tucked away in the brain cells.

Perhaps you might like to post your worked answers and we can offer some comments regarding how you are going about running the questions. Please give us the details of your thought processes rather than our trying to figure out what you are doing - I, for one, am not all that good at mind reading. Remember, I am a boring old pharte engineer who tends to see things in shades of black and white ...

Engineering specialist in aircraft performance and weight control.
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MissSoph replied the topic: PNR and trip fuel....

Hey John.... after seeking out help... I think I have the trip fuel definitions muddled ... I couldn’t grasp that trip fuel crosses over with fuel burn... so the equation asks me to effectively get fuel burn...but because the other question has fuel loaded in the tanks... and I need to take out fixed reserves variable... ect... the left over is effectively what I have to burn... trip fuel.... once I had grasp that.... I was all good.. well... a bit better... but I understand ... thank you for taking time out to help me... I do very much appreciate all of your time...
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John.Heddles replied the topic: PNR and trip fuel....

Think anticipated or expected fuel burn - that residual uncertainty is why we have reserves.

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