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When to use declared density

  • Aviatordan
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Aviatordan created the topic: When to use declared density

Gidday!!

I'm a little confused as to when you should use the declared density for the destination aerodrome when it comes to using the P charts to find landing performance.

Obviously it's used when no forecast is available but my question is relating to when there IS a forecast available.

The definition for forecast in the CAOs says "Atmospheric temperature and pressure conditions forecast for the aerodrome of destination, and its alternate if applicable, by an authorised meteorological officer with:
(a) 1 hour of the time of take-off of an aeroplane engaged on a flight within Australian Territory."


So does that "1 hour of the time of take-off" mean that if your flight has an EET of over 1 hour you have to use the declared density rather than the forecast?

Also, since wind is not mentioned in the definition and there is no such thing as a declared winds chart, does that mean you always have to use no wind for the calculation of landing performance?

Cheers guys!

Dan =]
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Richard replied the topic: Re: When to use declared density

Hi Dan,

You do have to consider temperature and pressure conditions forecast for the destination within 1 hour of take-off. However that means before take-off. You need to know if you're going to have a problem landing BEFORE you leave. So, that hour refers to an hour before wheels off.

Now, that doesn't mean you have to actually do your plan one hour before departure. You can still do your plan with the forecast conditions beforehand but just be sure to doublecheck the conditions before you go in order to ensure there has been no significant change that will affect any landing limitations that may apply to your flight.

As for using declared density altitude charts, you use them when either elevation, QNH or temperature are unknown at the destination. In that case you need to use the declared density altitude chart when determining if your flight is going to be landing weight limited.

This is something you could expect to do if you are planning a charter out to some farmstrip somewhere. It is unlikely you will have an authorised source of QNH or temperature and it is likely the strip length is going to be marginal. Using a declared density altitude will be necessary then to ensure you don't end up a statistic when you attempt a landing.

Cheers,

Rich
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  • Aviatordan
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Aviatordan replied the topic: Re: When to use declared density

Ahh, 1 hour before wheels off. So, it's saying that you must obtain the forecast within 1 hour before take-off, whether it be obtained at 60 minutes before or 15 minutes before, aslong as it's within an hour of take-off.

So, pretty much if you're flying into any aerodrome (or simple ALA) that doesn't have any form of forecast attached to it, you must use the declared density charts. (Unless it's the home on an authorised meteorological officer and he has the equipment!!)

So, do you always have to use no wind considering you're not allowed to use the wind in a forecast?

Cheers.
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Richard replied the topic: Re: When to use declared density

We're talking specifically here about using a forecast to work out if your maximum weight at take-off is going to be limited by a maximum landing weight at the destination. So, no earlier than an hour before take-off you should check the forecast to ensure the forecast conditions at the destination (QNH and temp) will still let you get in. It may well be the laterst forecast is the one you used to plan with and nothing has changed. The rule is all about checking again before you actually leave.

Yep you would have no choice but to use declared density altitude charts in the situation you mentioned. Of course in practice, you won't need to do this for all strips without a TAF. If the field is plenty long enough, the maximum landing weight will simply be the maufacturer's maximum landing weight for the aircraft.

You must not use wind from the forecast when determining if you can get into a strip or not. In other words, you are not allowed to look at the P-charts and say "Yeah, it's short. hot and high but I can get in because there's a forecast 20 kt wind down the runway". What happens if you turn up and the forecast wind doesn't materialise? Suddenly that strip is way too short. So, no winds when considering whether you can get in or not.

Just to be clear though, you do consider the wind when deciding if you need an alternate or not. If the forecast crosswind at the destination exceeds the permitted crosswind limit of the aircraft, you can't say "Bah, it's only a forecast so I'll ignore the wind". You'll need an alternate.

Cheers,

Rich
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Richard replied the topic: Re: When to use declared density

P.S. Another point for the exam: if a question asks you to calculate the landing distance required "under the following conditions" then they will expect you to use the wind in the P-chart.
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  • Aviatordan
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Aviatordan replied the topic: Re: When to use declared density

Hey Rich! Thanks for that! =]

So, my question now is that if you're not allowed to use wind in the landing P charts (except when it is given in an exam), why do they make allowance for it in the chart?

Oh, and also, in the exam, what happens if they say "calculate the landing distance required under these conditions..." but then give you a TAF to work off? Are you supposed to know that you're not to use the wind or do you use it anyway?

Sorry for asking so many questions!

Cheers Rich :)
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rubber replied the topic: Re: When to use declared density

Aviatordan wrote: ... why do they make allowance for it in the chart?


I second this query. Except for an exam type scenario, when would a pilot need the headwind and tailwind components of a P-chart?

Cheers, rubber
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  • Aviatordan
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Aviatordan replied the topic: Re: When to use declared density

G'day Rubber,

You use the wind components when calculating take-off performance.

However the only two explanations I can give as to why the landing P Charts make allowances for the wind component are:
1 - most of the aircraft we fly here in Australia originate from the USA and therefore the POHs are based on American regs which may be different to ours; and
2 - for an inflight calculation of landing performance at an aerodrome that wasn't planned for (assuming they have a TTF or METAR appended to them).

Other than that, we'd never use the wind components on landing.

Cheers =).

Daniel
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bobtait replied the topic: Re: When to use declared density

Note also that the CAO that says use no wind for a landing chart is actually referring to take-off limitations. If you are calculating a landing performance limit in order to establish a maximum take-off weight, then you should use no wind.

Makes sense when you think of it. You don't want to take off at a weight which requires a certain wind component at the destination aerodrome when you cross the fence for landing. If you plan on no wind, then any headwind that exists for landing will be a bonus.
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  • James B
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James B replied the topic: Re: When to use declared density

Hi Rich and Bob

I understand that for determining if we are landing weight limited we use no wind.

I also understand that if we are given wind, QNH and temp and told calculate the landing distance required under these conditions we can use the wind argument in the P Chart to accurately determine the landing distance in an exam.

RE: Aviatordans query a few months ago
"Oh, and also, in the exam, what happens if they say "calculate the landing distance required under these conditions..." but then give you a TAF to work off? Are you supposed to know that you're not to use the wind or do you use it anyway?"

So if they give me a TAF and say calculate the LDG distance under those conditions can I use TAF wind or should I use no wind what if it's a TTF? I assume ATIS wind to calculate LDG distance would be ok as long as you are not determining if A/C is LDG weight limited and were expecting to be landing in those conditions imminently?

Cheers for the great forum and support!
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