Performance calcs

[foodstamps]

Hi guys I hope somebody here can help me out as it has been a while since I have flown and my theory is rough

I am a little confused as to what information you are allowed to use when calculating performance information such as take-off & landing charts.

I am under the presumption (correct me if I'm wrong) that when calculating for an aerodrome / ALA with no information (ie TAF) that you would just use declared density charts and in the POH you would enter that density height as a PH along with what the ISA TEMP should be?

As the POH uses temp & PH to figure out distances I am presuming you should only use actually observed conditions (current ATIS). What's the go with using a TAF or TTF? Are they acceptable sources of information regarding temperatures / QNH for calculations or again should declared density charts be consulted?

Sorry if it seems like a basic question but it has been a few years.
Thanks

[The Green Goblin]

Declared density charts are for a planning stage, i.e you're in the office and want to work out if you can get into x next week for a charter.

On the day if you have an ATIS or AWIS, then this is the acceptable source for your performance calculations. If you don't, that is why it is a requirement to have an OAT gauge, so you can derive the temp and the pressure height from your altimeter for performance calculations.

If you look in CAO section 20, around 20.7 from memory there should be all the info you need.

I used to have a notepad in my nav bag with all the strips I visited. I made a worst case scenario figure (Temp 40, PH 5000, TW 10 knots, Max weight etc) then wrote the TO and LDG figures in my notepad and in my country airstrip guide for that field. Provided I was batter than that figure I was right to go.

This was also acceptable for the ATO when I did my CPL flight test which saved me a bucket of time in the limited planning time you receive.

[ConfigFull]

Gday,

The CAOs answer nearly all of your questions!

Firstly, in regards to using density altitude you are correct. CAO 20.7.1 Appendix 1 and Appendix 2 (pages 4-7 - http://www.casa.gov.au/wcmswr/_assets/main/download/orders/cao20/200701.pdf), Sections 2 in both describe how to use declared density on takeoff charts (it's a bit long-winded).

Secondly, you must use ambient conditions for takeoff calculations if you don't use declared conditions (CAO 20.7.0 Definitions http://www.casa.gov.au/wcmswr/_assets/main/download/orders/cao20/200700.pdf).

This means that a current ATIS (within the 15 minute limit of course) is acceptable (and is the easiest way), or you could just look outside at the windsock, set the aircraft's altimeter to 1013 and get the temperature either from the aircraft's OAT gauge or a thermometer, I believe there are a few requirements with the thermometer though that I can't find references for it).

Landing is a bit different, CAO 20.7.0 Definitions (same as above) instructs you to only use forecast temperature and pressure within the given time limits (domestic/international) - no mention of wind so use zero wind (worst case scenario).

Good luck with it all!

[foodstamps]

Do those CAO's apply? 20.7.1 at the top says for a/c above 5700kg MTOW... which the a/c I plan to fly are not

[ConfigFull]

CAO 20.7.4 is for <5700kg - my CAOs are still tagged for ATPLs so my bad.

That said if you read the pages I said you'll note that nowhere in Appendix 1 or 2 does it specify between <5700kg or >5700kg though - you would struggle to find a better step-by-step description of how to use the CASA issued P-charts.

I'm an instructor and from my experience everything that is on those pages is entirely accurate no matter what category.

[MakeItHappenCaptain]

You are correct,
The declared density is used if there are no observations/TAF available.

As you said, you enter the chart using the PA as DA and correct for ISA temp at that altitude. (Density equals Pressure corrected for temp, so if there is no ISA deviation, PA=DA):ok:

[foodstamps]

Ok thanks guys. Reading and understanding CAO's is always a fun activity :)

20.7.4 (d) gives me the impression (the way I am interpreting this) that even with forecast conditions being used (ie with the 1hour rule!) to ignore wind and treat it as zero wind.

20.7.4 Section 5 (Landing Weight Limitations) doesnt say anything in regards to forecast conditions being used. Just says that declared conditions may be used instead of actual PH and temps.

[SM227]

What about those crusty old P-charts written on parchment that are squashed to the bottom of the back seat pocket? You know, the ones that seem to have come from the CAA?
What are you s'posed to do with them? I thought the idea is that you either use that chart, or POH figures +%15, is that right?

[ConfigFull]

foodstamps, sorry to give you half an air law exam there - I just think it's better to see the actual legal references rather than rely on 'word of mouth' law which seems to run rampant in GA!

SM227 - you can still use those charts, they have the 15% figure already added so just read off the chart, otherwise as you said add 15% to the book figure - keep in mind this changes if you move up to anything above 3000kg.

Don't forget that in 20.7.4 is says you only use slope if it is in excess of 1%, catches people out on exams all the time.

[MakeItHappenCaptain]

ignore wind and treat it as zero wind.


Well if you take off into wind, you're only going to have better performance, then.
Worst case scenario wins.

[FGD135]

... you could just look outside at the windsock, set the aircraft's altimeter to 1013 and get the temperature either from the aircraft's OAT gauge ...


About that "just look outside at the windsock" bit ...

Can we, in the course of using ambient conditions for the purpose of calculating takeoff performance, just look at the windsock and estimate the headwind component?

I am curious about that. I have just had a good read through all the CAO 20.7 sections and it does not say anywhere that you can't.

I would have thought that, unless provided by a proper meteorological source, a zero headwind component should be used when applying ambient conditions.

That, to me, would be the "legally safest" way. Allowing pilots to "estimate" the headwind opens the door to overly creative values of headwind (or downwind).

What does everybody think?

[The Green Goblin]

Use a downwind and then you are covered for both :ok:

[ConfigFull]

FGD135 I completely agree, nil wind is always the safest. Like you I can't find (CAOs or AIPs) where it says you can't use the windsock for performance calcs - I suppose the entire purpose of the windsock is to provide bearing and strength (well not anymore but a rough figure) of the wind - otherwise we wouldn't have them!