Hi all,

To those that haven't heard, I failed Performance for the second time (very expensive and frustrating), so Richard came up with the idea to post some of my own questions and workings on here to see if I'm on the right track. Please feel free to do the same with questions you may be having trouble with.

Righto here we go, gonna start off with a couple of easy ones (I think)..

Find Distance and Time to ETP:

Distance to ETP:
Total Dist A->B = 350nm
Wind A->B = 25kt headwind
TAS = 150kt

My workings:

<-25kt wind

->125kt A->B GS
<-175kt B->A GS
350nm distance

(Total Distance x Ground Speed Home)/2 x TAS

(350 x 175)/300
=
61250/300
= 204nm

Distance to ETP is 204nm.

Time to ETP:

204nm @ Ground speed out
204nm @ 125kt = 98min

Distance and Time to ETP = 204nm and 98min.

Find Distance and Time to PNR:

Charter flight
Fuel at start up = 160L
S/Up & Taxi = 4L
Fuel consump = 33Lph
Fixed reserve @ 33Lph
TAS = 135kt
Wind = 15kt tailwind A->B

Find safe endurance:
160L
- 4L
- 25L (45min @ 33lph)
= 131L
131/1.15 (for variable res)
= 114L

114L @ 33Lph = 207min

so.............

(Safe endurance x GS Home)/(2 x TAS)
=
(207 x 120)/270
=
24840/270
= 92min

Time to PNR = 92min

Find Distance to PNR:

92min @ GS Out:
92min @ 150kt = 230nm
Distance to PNR = 230nm

Prove it:
92min @ 150kt = 230nm
(207min-92min = 115min)
115min @ 120kt = 230nm

Your ETP calcualtion is fine. Remember the proof for an ETP is to check the time from the ETP to the destination if you choose to go on. In this case the distance from the ETP to the destination is 350 - 204 = 146 nm. If you go on the time will be 146 nm @ 125 kt = 70 min. If you decide to come home the time will be 204 nm @ 175 kt = 70 min.

Also be careful that you read the question, because sometimes he asks for the distance from the ETP to the destination - which is not what the formula gives you.

Your PNR calculation is OK but your proof is a bit unusual. To prove a PNR we need to show that the time to the PNR plus the time back to base is equal to the safe endurance.

In this case:
Safe endurance = 207 min.
Time to the PNR = 92 min. Distance to PNR = 230 nm.
Time home from PNR = 230 nm @ 120 kt = 115 min.
Add time out to PNR and time back from PNR = 92 min + 115 min = safe endurance. Proved.

Ok thanks Bob,

So, 204nm TO the ETP (from A) is correct? But the time I calculated isn't. Am I supposed to get my answer from the time FROM ETP to B? In this case, as you stated 146nm@125kt=70min, THEN prove that it would take the same time to go back, ie. 204nm@175kt=70min.

I must be concentrating too hard on the first half of the problem, just the time and dist TO the ETP (from A) and not even looking at the rest of it.

And as for PNR, I'm using distance as my proof rather than safe endurance (which is the point of PNR calcs all along).. Now you put the finger on it, it makes more sense. Yep, gotcha.

Thanks! I'll be back when I have more to ask you,

Cheers

There is nothing wrong with the time you calculated. It would take you 98 mins to get to the ETP from the departure aerodrome. The 70 minutes to go on or back from the ETP is simply a PROOF that the answer you calcualted is correct.

If a questions askes for the time TO the ETP your calculation of 98 mins is correct, since you will be flying the 204 nm to the ETP at 125 kt.

Bob,

Quick question regarding balance and C of G's..

Now, I'm just going through Ex5.11 to do with calculating ballast fuel req'd. Just so it's clear to me, if I add weight forward of my C of G, that will bring the C of G forward, if I add weight aft of my C of G, that will bring the C of G aft?? Yes?

So if my C of G is 2710mm (using Q5 as an example), there is no point trying to add weight on any station that is further than 2710mm aft of the datum? eg. wing lockers or rear comp..

Sorry if this is painfully obvious, but from where I'm sitting I just need this clarification!

In this question, I added 10kg to the nose locker (which maxes that out) and got 2710mm. Now I still have 30kg of hard ballast to play with til I hit max ZFW, but there's just nowhere practical to put it because the only station ahead of my C of G is the main tanks!

From here I add ballast fuel to the mains, by doing a Weight to Add flow chart, which gives me 86.6kg (87kg).

Phew, according to the worked answers it's correct, thank god, I think I'm finally getting it

Yep, it looks like you're getting it!! Everything you have said above is correct. If you add what you can to the nose, there is no point adding any more hard ballast because all of the available compartments are behind the CofG so any weight added anywhere else but the nose or Row 1 would only make it worse. Row 1 is not available because it already has two occupants. All you can do is add ballast fuel. This type of question always puts you in that situation. You add what you can to the nose, and although that makes it better it doesn't fix the problem. Your only option is to add ballast fuel. Well done mate!!

Righto, another couple of exercises..

An Echo is loaded as follows:

Weights in kg's, Moment Index kg/mm in brackets

Row 1 154kg (35.26)
Row 2 154 (50.82)
Row 3 150 (64.5)
Nose 15 (.75)
Rear 100 (50)
BEW 1975 (498.9)

Gives us a total weight of 2548kg (700.23) resulting in a CofG at 2748mm (out).

If we now add 40kg to the Nose

Nose 40 (2)

This brings it up to 2588kg (702.23) and a CofG at 2713mm (still out).

So through the Weight to Add flow chart it works out to be 94.9 (95kg) of ballast fuel to add to the mains to put the CofG right on the aft limit.

Without being able to see the flowchart I made up and not being able to look at any answers, are there any errors in my working?

Also while I'm at it, I've been getting into the forward limit stuff, and the formula is from memory, ((current weight-2630)x.27)+2400 sooooooo say I have a weight of 2795kg. The forward limit is 2444.5mm? And if I want to add or subtract weight to put the CofG on the forward limit, I must use the graph to work out how much?

Thanks in advance

Your fuel as ballast calculation is spot on. No problems there.

The forward limit formula is Present weight - 2360 [not 2630]x .27 + 2400.

Remember that the formula solution for forward limit questions only works if you are SHIFTING weight, because when you shift weight you are not changing weight. If you are adding or subtracting weight to get to the forward limit, you must use the envelope and plot the two points, join them, and estimate the amount of weight that must be added or subtracted. Se the text book page 5.49.

Keep having fun!!

ahhhh OK I did actually have that written down but thought it must have been a typo on my part because "why would it be 2360 when talking weights" (as 2630 is the max ZFW..) haha. No worries, I'll just remember it's the opposite of what I think and that should do it.

Got it, thanks Bob!

Actually 2360 kg is the point below which the forward limit remains steady at 2400 mm. See text book page 5.47