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## PNR and ETP - Q9 and 10 page 4.51

• brentonrule
• Topic Author

### brentonrule created the topic: PNR and ETP - Q9 and 10 page 4.51

Hi all.

I am struggling a bit with these questions.

Q9 is clear that (c) is correct as you say "any wind will cause PNR to be closer to the departure aerodrome"

Q10 by that logic with a lesser tail wind (less g/s out and more g/s home relatively) means that (by my logic) the ETP moves out (due to a faster INBOUND speed), and the PNR moves closer to home (due to any wind) making the answer (c).

This is not correct however; the answer is (a) ETP closer and PNR further.

Thanks again.

• brentonrule
• Topic Author

### brentonrule replied the topic: Correction to my submission

Woops, the answer in a) is Both ETP and PNR are further - and this is my area of confusion - ie: any wind should mean it is closer?

• brentonrule
• Topic Author

### brentonrule replied the topic: PNR and ETP Calcs

Hello Richard and others. Please note I have since done the long hand calcs and I now get the correct answers. The ETP/PNR logic with a lesser tail wind is still a bit hard for me to grasp. ETP I can get but the PNR result still surprises me.... Oh well.... Anyway, no need for a detailed answer on this one for me - others may benefit though.

Into the home stretch doing payloads and CofG now - overall I am really enjoying this curriculum, your books and methods make the complex seem simple

• Posts: 1169

### Richard replied the topic: Re: PNR and ETP Calcs

Hi Brenton,

The basic rules are:

PNR is furthest out in no wind. The stronger the wind the closer the PNR is to base.
ETP moves upwind from halfway. The stronger the wind, the further the ETP point moves upwind.

In Q10 we're told we have a tailwind. That tells us the PNR must be closer to base and how much closer is dependant on the strength of the tailwind. We find the tailwind is less than expected so the PNR point must move less distance towards base. The net effect is the PNR is further out from base than we expected.

As for the ETP, we know it moves upwind from halfway. We have a tailwind so we know it must have moved upwind and therefore back towards our departure point. However we find we have less of a tailwind than we expected so the amount of movement of the ETP upwind is less than we originally expected. That means the ETP is also further out from base than we anticipated.

Final answer, both the PNR and the ETP will be further from base than what was originally planned.

As you found out, you can check that with calculations but you can save yourself some time by just remembering the rule of thumb:

PNR moves closer to base with ANY wind.
ETP moves upwind from halfway.

Of course, everyone should make sure they understand WHY that's true before blindly remembering rules

Cheers,

Rich