×
##### PPL Video Lectures (10 Jul 2020)

PPL Video Lectures covering Aerodynamics, General Knowledge, Performance, Meteorology And Navigation are now available through our website see front page for details.

× Welcome to the CPL Performance question and answer forum. Please feel free to post your questions but more importantly also suggest answers for your forum colleagues. Bob himself or one of the other tutors will get to your question as soon as we can.

## Alternate + INTER fuel requirements and other fun questions

• Posts: 8
• Thank you received: 0

### phlegm created the topic: Alternate + INTER fuel requirements and other fun questions

Just walked out of my first attempt at this exam with a 65%. Really thought I had it nailed so pretty disappointed. Couple of questions that threw me:

1. Alpha to Bravo, where Bravo requires alternate fuel to Charlie, and Charlie has an INTER. Find fuel required at engine startup for a day VFR private SE piston. I did taxi + 30 min FR + trip fuel + alt fuel + 30 min hold. Was the hold fuel unnecessary? I've not seen this combination before.

2. Effects of wind on the CP. They said you have calculated a PNR in nil wind conditions. What effect will an increase in forecast winds have on the time to return from or continue onwards from the PNR? So I know that the distance to the PNR is greatest in nil wind, and that either a HW or a TW component will decrease the distance to the PNR. I ran the calculation with nil wind and again with an arbitrary HW component with the same safe endurance for both, and found that the time to the PNR increased and the time to return decreased compared to nil wind. Answered as such. Showed up on my KDR. Confused about this one, my only idea is that I must have missed something in their needlessly convoluted language and got the answer backwards.

3. Rate of Climb required to achieve 6%. For this, they direct you to the Echo take-off chart. I was given a weight, DH and headwind, but not the TODA or slope. I took the DH and wind to be red herrings and just used the VTOSS for the given weight. Is this correct, or should I have assumed a particular runway length so I could get around the p-chart?

Thanks.

Please Log in or Create an account to join the conversation.

• Posts: 8
• Thank you received: 0

### phlegm replied the topic: Alternate + INTER fuel requirements and other fun questions

Anyone?

Please Log in or Create an account to join the conversation.

• John.Heddles
• Offline
• ATPL/consulting aero engineer
• Posts: 624
• Thank you received: 67

### John.Heddles replied the topic: Alternate + INTER fuel requirements and other fun questions

1. Alpha to Bravo, where Bravo requires alternate fuel to Charlie, and Charlie has an INTER.... day VFR private SE piston.

Start with a review of CAAP 234-1(2.1) at www.casa.gov.au/sites/default/files/caap...or-aircraft-fuel.pdf

4.1.1 requires, for a small aircraft on Day VFR PVT operations, 30 mins fixed and no variable reserves.

4.2.1 requires you to carry holding fuel if that be required for your flight - was the hold fuel unnecessary? If holding is required, then you require the fuel to be able to do that hold (should it turn out to be necessary when you get there).

I did taxi + 30 min FR + trip fuel + alt fuel + 30 min hold.

Sounds about right to me ... although this conservative airman would probably take a bit more, depending on how I interpreted the wx forecast. One must keep in mind that the rules give the minimum solution, but not always the sensible solution. That's where training, experience and common sense play their role in operational management.

2. Effects of wind on the CP

As often is the case, it's a bit difficult without seeing the actual question.

However, if we look at the basic CP formula

Dcp = (total distance x GS home) / (GS home + GS on)

and the CP position moves into wind from the half way point between departure and destination.

A problem occurs in real winds (and one which we tend to ignore) where the magnitude of the crosswind can muddy the waters a little, so let's assume, for this discussion, that we are talking pure HW or TW so that drift considerations don't come into play and we get a basic sort of indication as to what's going on. This will be adequate until wind speeds and drifts become significant and will cover the questions you are going to come up against in the CASA exams.

With this caveat, the equation simplifies to

Dcp = (D x (TAS - W)) / ((TAS - W) + (TAS + W))

If we play around with that, just a little bit, we get

Dcp = (D x (TAS - W)) / (2 x TAS)

Now, if we look at the difference between the nil wind and wind cases, we can come up with

delta Dcp = Dcp nil wind - Dcp wind = (D x TAS) / (2 x TAS) - (D x (TAS - W)) / (2 x TAS)

which simplifies to

delta Dcp = (total distance x wind component) / (2 x TAS)

Now, we can play around with this a bit further and come up with

delta Dcp = ((total distance / 2 ) / TAS ) x wind component

It might make more sense to put this into words, as - in a wind, compared to nil wind (or, if the wind strength increases) -

the distance to the CP moves into wind from the half way point by (half the total distance x wind component/TAS)

and you will see versions of this in various texts about the place. It then is a simple matter to figure out the time interval home or on from the CP.

3. Rate of Climb required to achieve 6%.

We covered this to some extent in a recent thread at bobtait.com.au/forum/performance/6784-mi...mb-rate?limitstart=0

I was given a weight, DH and headwind, but not the TODA or slope.

Weight and PH/OAT (DH should be OK) are necessary. HW may be or may not be, as discussed in the other thread. TODA and slope are irrelevant.

I ... just used the VTOSS for the given weight.

What "sort" of speed is Vtoss ? What sort of speed do you need to run the gradient calculations ?

Is this correct, or should I have assumed a particular runway length so I could get around the p-chart?

Not necessary, as the only interest you have in the P-chart for this question is to get the takeoff safety speed.

Now, just get back on the horse, back into the books, and knock over the exam next time around ...

Engineering specialist in aircraft performance and weight control.

Please Log in or Create an account to join the conversation.

• Posts: 8
• Thank you received: 0

### phlegm replied the topic: Alternate + INTER fuel requirements and other fun questions

Thanks John. I just read that thread about the climb gradients and can see my error: I needed to convert the TOSS (which is an IAS) into TAS using the density height, then into a GS using the winds. It's obvious in hindsight, I've just never seen that kind of question presented with a P-chart, so I didn't grasp what they were asking for.

Sounds like I had the right approach with the INTER + alternate requirements, so it must have just been a silly arithmetic error somewhere along the line.

I'm still scratching my head trying to decipher everything you've said about wind. My confusion stems from the term "critical point", as I gather that term has been retired (but is still mentioned in the MoS). My question was about PNRs, not ETPs, and I think you're using the formula for the latter (ie using distance instead of safe endurance). But equally possible that I'm just confused.

Please Log in or Create an account to join the conversation.

• John.Heddles
• Offline
• ATPL/consulting aero engineer
• Posts: 624
• Thank you received: 67

### John.Heddles replied the topic: Alternate + INTER fuel requirements and other fun questions

I needed to convert the TOSS (which is an IAS) into TAS using the density height,

That's the story. Just be careful to keep in mind that IAS to TAS usually will give you a PEC error and a small TAS error The correct sequence is IAS to CAS to EAS to TAS. For the exam animals, however, we have no PEC chart data, EAS is irrelevant due to the low Mach number, so we are forced to use the (inappropriate) IAS to TAS conversion. In the real world of light aircraft you would have the PEC data in the POH so you can do the exercise correctly.

then into a GS using the winds.

Take care. 6% is a WAT limit and, as such, applies without regard to wind - ie it is a nil-wind consideration. However, I have seen questions couched in a generic form where the intent was to include wind. Probably, some folks just don't understand the history and what the 6% is all about. The examiner, though, could well ask questions of either ilk to test the candidate's underlying understanding. At some stage I must run the question past him to see just what is the exam situation. Some more thoughts later in this post as it is an important consideration and, unfortunately, very poorly understood by lots of folks in the pilot game.

My confusion stems from the term "critical point", as I gather that term has been retired (but is still mentioned in the MoS).

I must confess that I hadn't heard anything about retiring of terms. There are several terms applicable to the concept; critical point (CP), equi-time point (ETP), and so on. CAAP 234-1(2.1) refers to these as well as equal time point. It doesn't really matter what you call it so long as you understand what the requirement is.

My question was about PNRs, not ETPs, and I think you're using the formula for the latter (ie using distance instead of safe endurance).

Hence my earlier comment about preferring to see the specific question. However, as you described things, it was reasonably clear that we were talking about CP style consideration, regardless of how the initial position might have been arrived at. In particular, your references to

(a) effects of wind on the CP

(b) what effect will an increase in forecast winds have on the time to return from or continue onwards from the PNR

strongly suggested that the references to PNR were just a sideshow consideration.

Possibly a cleverer than some sort of question designed to test basic understanding as opposed to rote learning ? In particular, the statement at (b) suggests that the question was setting out to do a bit of candidate testing ?

Thoughts on the 6% thing

This is a WAT limit and comes from the long ago cancelled CAO 101.22 in the airworthiness series. 20.7 is an operational CAO and, historically, quoted lots of stuff from the various airworthiness rules.

If we look at 20.7.4 (which you can find at www.legislation.gov.au/Details/F2014C01367 ) section 4.1 (b) tells us that one of the limits relates to

a weight which will permit compliance with the take-off climb requirements specified in subsection 7 taking into account ambient temperature and pressure height.

7.1 then brings up the 6% thing.

You will note that the requirement involves Hp and OAT (which usually, but not always with these charts, is Hd) but NOT wind. This is entirely consistent with the philosophical idea behind WAT limits that we are looking at "line in the sand" maximum weights to ensure a modest, reliably anticipated climb gradient.

Perhaps we can take a look at a typical WAT limit presentation, eg in the exam workbook (which I'm sure you have but can be found at www.casa.gov.au/sites/default/files/rpl-...roplane-workbook.pdf ). If you check page 5 there is a takeoff weight chart (for whatever aircraft - the chart would have been drawn up many, many years ago for the relevant old-style Civil Mk 1 or Mk 2 DCA flight manuals) you will observe that it has a climb limit line in the bottom left hand (density height carpet) graph. This line is the 6% limit line meeting the old 101.22 requirement (as restated in 20.7.4.7.1). You will notice, quite clearly, that the limit line has NO PROVISION for any wind entry - it only considers Hp and OAT, as required by 101.22 and 20.7.4.

The same observation may be made for the takeoff chart on page 7.

Mind you, many things, exam-wise, will be changing with the introduction of the new Flight Operations rules in early December 2021.

Engineering specialist in aircraft performance and weight control.

Please Log in or Create an account to join the conversation.

• Posts: 8
• Thank you received: 0

### phlegm replied the topic: Alternate + INTER fuel requirements and other fun questions

Thanks for the tip about using nil wind, that probably would have tripped me up!

Please Log in or Create an account to join the conversation.

• John.Heddles
• Offline
• ATPL/consulting aero engineer
• Posts: 624
• Thank you received: 67

### John.Heddles replied the topic: Alternate + INTER fuel requirements and other fun questions

tip about using nil wind

Be cautious and read/interpret the questions carefully. By rights, you should expect to use nil wind but, as I suggested earlier, the question might just infer the intent to incorporate wind.. If the question clearly is linking itself to 20.7.4, nil wind, if couched in terms of generic gradients out there for a routine takeoff, you need to make the call whether you run with/without wind.

Engineering specialist in aircraft performance and weight control.

Please Log in or Create an account to join the conversation.

Time to create page: 0.210 seconds