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## PNR Safe endurance

• Posts: 54

### Airspeed created the topic: PNR Safe endurance

I was checking out a PNR question on PPE.

From what I've learned in Bobs course we calculate the fuel for safe endurance as (FOB-Taxi-Startup-Fixed)/1.1

This person did the fuel for safe endurance differently and did not divide by the 1.1, like above, but Instead calculated the variable reserves separately as a function of time to destination and fuel burn outbound, then took 10% of that and subtracting that from FOB-startup-taxi-fixed.

Doing that gives a totally different answer, so which method is to be used in the exam?

• Posts: 2261

### bobtait replied the topic: PNR Safe endurance

Perhaps your friend took 10% of the total fuel as the variable reserve. For example:

Total fuel on board = 75 gallons
Less 10% of 75 gallons =7.5 gallons)
Less Fixed Reserve 15 gallons
Less Taxi 3 gallons
= 49.5 gallons.
That is incorrect. The variable reserve is 10% of the TRIP FUEL. So you need to take out the fixed reserve and taxi first, that will represent 110% of the trip fuel. then take 10% then divide by 1.1 to remove the variable reserve.

For example:
Total fuel on board 75 gallons
Less fixed reserve 15
Less taxi 3
= 57 gallons. (110% of the TRIP FUEL) )
So divide by 1.1 to find 100% of the trip fuel
= 51.8 gallons

• Posts: 54

### Airspeed replied the topic: PNR Safe endurance

Your right, but I think what I'm seeing in the answer to the question I saw is the difference between calculating variable from the FOB v calculating it from the distance/GS x Fuel Flow.

EG
Day VFR Charter flight A - B, departing at 0100 UTC 193 nm to B, HDG 250M. Variation 10E.

TAF A 102321Z 1100/1124
220044KT 9999 - SCT030
INTER 1101/1106 SHRA BKN 010
T12 14 12 10 Q 1004 1002 1004 1006

TAF B 102321Z 1100/1124
220044KT 9999 - SCT040
T12 14 12 10 Q 1004 1002 1004 1006

FOB 320 L
Startup 4 L
Taxi 6 L
Cruise Fuel Flow at 130TAS = 76 L
Holding FF @90TAS = 56 L

Determine time and distance to PNR

GSo = 94
GSh = 162

INTER @ A = 30 min = 29L

Flight time A - B = 193/96 = 121 min

Fuel
Safe endurance = FOB - Startup - Taxi - holding
= 320 - 4 - 6 - FR(45/60x56) - VR (10%) - 30/60x56
= 268 - VR (121minx10%x76L) - 28
= 268 - 15.2 - 28
= 225 L

The VR calculation based on time and distance rather than removing all reserves and holding from the FOB, then dividing by 1.1 is where the difference is.

If you just do (320 - 4 - 6 - FR42 - HOLD28)/1.1 you get 218 L

The method I'm used to from your books is the latter method, not using the distance, time and fuel flow to work out the 10%.

Hope that makes sense.

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 670

### John.Heddles replied the topic: PNR Safe endurance

A bit hard to decipher just who did what calculations. However ...

Whoever did the calculations in the main block (I presume not your good self) looks to have been well and truly away with the fairies. Perhaps we all should have some of what he/she was smoking so we can all see the same pretty pictures in our various minds' eyes and ease our aches and pains ?

The material is riddled with a variety of errors. Presumably you detected these as you worked through the sums ?

Presuming that you are most interested in the text highlighted in red, it is totally irrelevant and has nothing to do with the PNR calculation. Had it been run sensibly the answer would have come out to be the same as Bob's although why one would go to that trouble is a bit beyond me.

The purported solution is one of the more complicated ways I have seen to arrive at a nonsensical answer ..... refer back to whatever the author had been smoking at the time ...

Being the slow, old chap that I am, I prefer not to learn any formulae I can avoid, so I just run ALL fuel sums as the standard fuel table - fill in the bits I have, or can work out easily, and then figure out the bits I don't have and/or might take a bit more thought. Seems to have worked OK for the past 60-odd years or so (from bugsmashers to Boeings), involves a minimum of brain strain, and I only need one technique to remember.

For what it's worth, my answer for the available fuel (with which one might play PNRs) is the same as Bob's. Forget the other "solution" altogether as it's just aberrant nonsense.

Engineering specialist in aircraft performance and weight control.

• Posts: 118

### Bosi72 replied the topic: PNR Safe endurance

>The method I'm used to from your
>books is the latter method, not using
> the distance, time and fuel flow to
> work out the 10%.

PNR is only about Safe Endurance, i.e. how many minutes you can fly without touching reserves. It depends only on amount of fuel in your tank and fuel flow. Nothing else. Distance A to B is irrelevant and likely misleading in the question, therefore using 121min in formula below is not correct.
VR (121minx10%x76L)

I also found some inaccuracies:
INTER @ A = 30 min = 29L
however it has been corrected to 28 in calculations below.

Also my e6b could be wrong, but I am getting GSo=93 and GSh=161

EDIT:
haha looks like John was a couple of minutes quicker than me.

• Posts: 54

### Airspeed replied the topic: PNR Safe endurance

Thanks guys. This is what I was thinking. I love Johns response that the person must have been smoking something.

I'm gonna steer away from the PPE test questions for this topic. There's numerous examples where they have implemented this type of answer and some where they have done it the "Bob Tait" way, just because they think many are taught that way, but if CASA expects the answer to be gained using the method Bob's book teaches, then whoever is pushing these type of answers for subtracting or adding variable reserves, should stop teaching the wrong method, because its just plane confusing.

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 670

### John.Heddles replied the topic: PNR Safe endurance

There's numerous examples where they have implemented this type of answer and some where they have done it the "Bob Tait" way, just because they think many are taught that way, but if CASA expects the answer to be gained using the method Bob's book teaches, then whoever is pushing these type of answers for subtracting or adding variable reserves, should stop teaching the wrong method, because its just plane confusing.

I suggest that CASA doesn't expect "Bob's method" - rather a suitable method to obtain the correct answer. Bob's solution happens to be one of these so it's fine. Often there be several ways to skin a cat, as the old saying goes.

The problem with the question you have cited is that the solution is JUST PLAIN WRONG ! If such solutions are common from this provider, it doesn't say much about the quality control within the provider's systems or, indeed, perhaps the basic competence of the provider's personnel ?

Engineering specialist in aircraft performance and weight control.

• Posts: 54

### Airspeed replied the topic: PNR Safe endurance

Totally agree. Thanks.

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 670

### John.Heddles replied the topic: PNR Safe endurance

This thread rather intrigued me ref the provider question and the previous comments, so I had a look at their website.

Interesting, but I have to stay with my previous comments at this stage.

While I was not able to review any of their main question banks, the walk up spectator (ie, me) is able to have a look at several sample sets of questions. I presume that these form part of their marketing program. Accordingly, I would have expected that these question sets would have been super polished for obvious marketing reasons.

From the sample sets I had a look at, my view is that what the provider is endeavouring to provide is good and should be a useful tool for candidates. However, that impression is very much spoiled when significant errors get through their checking systems. I am not concerned with the typical "not quite correct" things which are endemic in the Industry but, rather, straight out significant wrong stuff.

For instance,

(a) describing moments as distances from the datum point indicates a total lack of understanding of moment calculations which are the underpinning basis of weight and balance.

(b) confusing obstacle clearance requirements (5% gradient per CAAP 92-1(1) 5.5) with the certification takeoff climb requirement of 6%. This came from CAO 101.22, now defunct, but the operational copy in 20.7.4 still exists.

(c) one question appeared to be a confusion of takeoff run (which has little relevance to light aircraft - it is, on the other hand, quite critical for heavy aircraft), takeoff distance, obstacle clearance requirements and what the POH TODR data might be giving the pilot. Indeed, I was not too sure what to make of the question and solution.

The material would be great if one could have a higher confidence that it was all correct and subject to detailed quality control protocols in its preparation.

One hopes that, in time, the provider fixes these sorts of problems - their product should then be quite useful.

Engineering specialist in aircraft performance and weight control.