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## Qnh from a TAF

• Flyingpom
• Topic Author

### Flyingpom created the topic: Qnh from a TAF

I haven a question re QNH from a TAF. In your met book it states that the QNH values are valid for 90 min intervals so if we have a forecast from 0200 the TAF QNH values reading 1010 1011 1012 1013

Time

0200 - 0330 is 1010
0330 - 0630 is 1011
0630 - 0930 is 1012
0930 - 1230 is 1013

Now there is a question in the perf book ( Page 3.28 ) that states the QNH at time 0705 is 1009. From the logic described above shouldn't it be 1008??

• Aviatordan

### Aviatordan replied the topic: Re: Qnh from a TAF

It appears you're right, it's a mistake in the print! Ohwell, it happens to the best of us.

Considering it only makes a 30' difference and it's hard to be THAT accurate when doing these performance charts it doesn't really affect it, espeically since it's only an exercise.

Cheers!

• Posts: 1914

### bobtait replied the topic: Re: Qnh from a TAF

It's not a typo actually. The AIP used to say that the temperature and QNH information was presented as four values, each valid for 90 minutes before and after the commencement of the TAF validity period and at three hourly intervals after that. That presentation was changed with the last revision of the AIP.

The temperature and QNH information is now presented as four 'spot' values starting at the commencement of the validity period and then at three hourly intervals. To get a value at any other time, you should interpolate between the 'spot' values.

I have amended the text books to reflect the new presentation, but unfortunately I can't do anything about the books that are already in circulation.

• Aviatordan

### Aviatordan replied the topic: Re: Qnh from a TAF

That's silly, I had just mastered using the old method!

So, in the example on P3.28 of the performance book the TAF says:
Valid 2202/2214
30 32 33 30 1009 1009 1008 1007

It's now:

Time 0200 0500 0800 1100
Temp & QNH 30 & 1009 32 & 1009 33 & 1008 30 & 1007

So rather than the values being valid for 90 mins either side of the times they are now only valid for the 'spot times' and you now have to interpolate between them?

Cheers!

• Aviatordan

### Aviatordan replied the topic: Re: Qnh from a TAF

They didn't really stay in the format I had wanted! It's now sort of a match the times with their values, although they are in order.

• Posts: 1169

### Richard replied the topic: Re: Qnh from a TAF

Yep, that's how it works now. Here's the time to temp/QNH values you were describing.

TAF YCGO 160045Z 2202/2214 10015KT 9999 FEW020
FM 220500 17015KT 9999 BKN012
30 32 33 30 1009 1009 1008 1007
 TIME TEMP QNH 22 0200 30 1009 22 0500 32 1009 22 0800 33 1008 22 1100 30 1007

For intermediate times you simply interpolate as required and obviously 2214 will be read off the next TAF.

Cheers,

Rich

P.S. Just as a nerdy tip, if you want to tabulate values in these forum posts, there's a "Create embedded table" button in the toolbar immediately above the smileys. The tags
`[tr] and [/tr]`
define rows in the table and each cell is defined by
`[td] and [/td]`
You put the values between the td's and to add more cells to a row, add more td tags and to add more rows, add more tr tags. Easy eh

• Aviatordan

### Aviatordan replied the topic: Re: Qnh from a TAF

Funnily enough, I actually prefer the other method. At least it gave you the values you need and there was no educated guess work involved.

Ohwell.

And thanks for that tip! Guess I'm still learning how to fully use the forum!

Oh, and sorry for the hasty 'it's a typo' response, Bob. I was under the impression that it was still being worked out the old way! And I guess that's true for flying pom as well. I literally got a box full of updates the other day, haven't had time to look at them yet!

Cheers!

• Posts: 1169

### Richard replied the topic: Re: Qnh from a TAF

Aviatordan wrote: Funnily enough, I actually prefer the other method.

Dan, you are officially mental

Seriously though, we've been shaking our heads about the +/- 90 minutes rule and breathed a great sigh of "Thank god for that" when the AIP changed. To me at least it feels more intuitive to interpolate between spot values. It's something you can do in your head rather than grabbing a bit of paper and checking +/- 90 minute ranges in the old format.

The linear interpolation may seem to be a pain but really it isn't something likely to cause a problem. No one is going to interpolate to decimal places and in nearly all cases it is immediately obvious what the intermediate values are going to be.

The funny thing is, this whole thing revolves around guesstimates made from a forecast based on best guesses

Cheers,

Rich

• Aviatordan

### Aviatordan replied the topic: Re: Qnh from a TAF

Haha, thanks Rich!

I guess I like the other method because it plagued me when I was first taught how to use it when I was doing my PPL a couple of years back. I had just finally gotten familiar with it and they take it away!

I guess you're right about it being easier to calculate in your head, though! It'll make things easier in the cockpit, that's for sure.

And you're right, the term forecast pretty much by definition is the weatherman's best guess.