Performance Charts - Pressure Altitude conversion with Foreflight?

Open for questions of visitors of DAN. Posts of our guests are on moderation queue.

Moderators: Rick, Lance Murray

User avatar
ingramleedy
3 Diamonds Member
3 Diamonds Member
Posts: 92
Joined: Wed Apr 13, 2022 11:59 pm
First Name: Ingram
Aircraft Type: DA40NG
Aircraft Registration: N238PS
Airports: KBOW
Has thanked: 112 times
Been thanked: 51 times

Performance Charts - Pressure Altitude conversion with Foreflight?

Post by ingramleedy »

Hi all,

In most of the METARs or ATIS information I always hear "Density Altitude" and its calculated for you. I suppose this is a big reminder to check your performance charts - especially on hot, humid, short runway days.

All the Diamond Performance charts are based on Pressure Altitude with a temperature chart for its corresponding "density altitude performance numbers". Now I know you can simply calculate pressure altitude with a formula. For example: Let’s say the current altimeter setting is 29.45 and the field elevation is 5,000 feet. Plugging these numbers into the pressure altitude formula, you get: (29.92 – 29.45) x 1,000 + 5,000 = 5,470 feet. Which then I can use in my charts that get me to temp, and then finally to the actual performance number.

It seems that ForeFlight should also show you the pressure altitude in the METARS too so we can work it easier into these charts or if the charts showed the density altitude. (lazy I know) I also know I can use the Performance Plus in ForeFlight and look at Takeoff and Landing performance, which probably calculates humidity too and is a good check but it does take a little more setup too.

Maybe I am overthinking this ... But does anyone have any thoughts on this? What do you do when you are planning?

-Ingram
User avatar
Boatguy
5 Diamonds Member
5 Diamonds Member
Posts: 1827
Joined: Mon Jan 22, 2018 4:48 am
First Name: Russ
Aircraft Type: DA62
Aircraft Registration: N962M
Airports: KSTS
Has thanked: 1328 times
Been thanked: 1163 times

Re: Performance Charts - Pressure Altitude conversion with Foreflight?

Post by Boatguy »

In the NG, the variation across altitude and temps is so small that I don't give it a lot of thought. With my wife, my TO weight is basically the 2,645lb table +-50lbs. The difference between SL at 10C (home) and 6,000' at 30C (Santa Fe mid-morning) is 90fpm. I assume I'll climb at 600-700fpm.

What I try to anticipate is the potential impact of downdrafts (the updrafts are all good). While I know I'll average 650fpm, a minute of being pushed down to 200fpm can be an issue.
User avatar
Rich
5 Diamonds Member
5 Diamonds Member
Posts: 4592
Joined: Mon Aug 30, 2010 12:40 pm
First Name: Rich
Aircraft Type: DA40
Aircraft Registration: N40XE
Airports: S39 Prineville OR
Has thanked: 145 times
Been thanked: 1180 times

Re: Performance Charts - Pressure Altitude conversion with Foreflight?

Post by Rich »

FWIW FF does display density altitude in METARS.
FF will display your current Pressure Altitude from 2 different sources in the instruments panel if you configure one or both for display.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
User avatar
Rich
5 Diamonds Member
5 Diamonds Member
Posts: 4592
Joined: Mon Aug 30, 2010 12:40 pm
First Name: Rich
Aircraft Type: DA40
Aircraft Registration: N40XE
Airports: S39 Prineville OR
Has thanked: 145 times
Been thanked: 1180 times

Re: Performance Charts - Pressure Altitude conversion with Foreflight?

Post by Rich »

I've been noodling the idea of creating a more usable tool for my DA40-180 based on the performance charts. Since Density Altitude is most readily available for airports, it would start from there, rather than with PA+temperature. I haven't given it a priority, though, for a number of reasons. Mostly it's not worth worrying about unless I'm departing at high DA with obstructions around and unusually heavily loaded. If I'm really traveling any distance it'll be me and my wife with some baggage, so I'm no heavier than 2300 lb. at TO.

Another other issue is that, other than takeoff, the remaining performance charts have so much in the way of assumptions about atmospheric conditions that one can't take them for any more than approximations. I base my planning and real-time decisions on long experience.

Having said that, for those new to their plane it's worth working out some scenarios ahead of time, recognizing that there is a fairly large uncertainty envelope, especially with respect to climb.
2002 DA40-180: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
Post Reply