Density altitude and runway length quick guide?

jspilot

jspilot

Cleared for Takeoff
Joined
Oct 22, 2011
Messages
1,346
Display Name
Display name:
jspilot
So I’m a flatland pilot guy who has almost no experience with density altitude so forgive the ignorance. When I was training my CFI’s admittedly did not spend a lot of time on density altitude but on a recent flight I was confronted with a situation I was not sure of.

I went to Pittsfield Mass(PSF) which is up about 1,000 feet above sea level. Winds favored the shorter runway( 3,500 feet). Landed with no issue but was just making a quick stop. Listened to the weather and it was reporting density altitude of 2,800. That’s higher than I’m used to( I know many are probably laughing if you fly in the mountains) but it occurred to me that the runway was kind of short and the density altitude was high(ish) and it was warm so I decided to do a short field takeoff. It worked fine and I probably took up about half the runway to get airborne.

It occurred to me though, that I really had no way to quickly judge runway length vs density altitude and was wondering if anyone had a quick guide or “rule of thumb” they use in a similar situation. I’m glad I did the short field takeoff but really was not sure how I should know for sure( without taking the time to break out the handbook) if I actually needed to do one. Does density altitude within 1,000 feet of runway length work as a quick reference for when a short field takeoff should happen? Does anyone have a rule for this?

Clearly I should have looked at the handbook but my knowledge in this area is admittedly not as strong as it should be.

Thanks for the help!
 
jspilot said:
So I’m a flatland pilot guy who has almost no experience with density altitude so forgive the ignorance. When I was training my CFI’s admittedly did not spend a lot of time on density altitude but on a recent flight I was confronted with a situation I was not sure of.

I went to Pittsfield Mass(PSF) which is up about 1,000 feet above sea level. Winds favored the shorter runway( 3,500 feet). Landed with no issue but was just making a quick stop. Listened to the weather and it was reporting density altitude of 2,800. That’s higher than I’m used to( I know many are probably laughing if you fly in the mountains) but it occurred to me that the runway was kind of short and the density altitude was high(ish) and it was warm so I decided to do a short field takeoff. It worked fine and I probably took up about half the runway to get airborne.

It occurred to me though, that I really had no way to quickly judge runway length vs density altitude and was wondering if anyone had a quick guide or “rule of thumb” they use in a similar situation. I’m glad I did the short field takeoff but really was not sure how I should know for sure( without taking the time to break out the handbook) if I actually needed to do one. Does density altitude within 1,000 feet of runway length work as a quick reference for when a short field takeoff should happen? Does anyone have a rule for this?

Clearly I should have looked at the handbook but my knowledge in this area is admittedly not as strong as it should be.

Thanks for the help!
Click to expand...

You basically have 6 variables that effect takeoff performance - Density altitude, runway surface, runway grade, headwind component, humidity, and aircraft weight. If you look in the performance tables for you aircraft, you should easily be able to determine a hard level surface, calm wind, gross weight distance for a DA of 5000 ft plus 50% margin of safety about any situation you have in the flat lands.
 
It’s not predictive but a “last check” rule of thumb - the 1/2-2/3 rule = by halfway down the runway you should be at least 2/3 of the airspeed needed to get airborne.
 
You must log in or register to reply here.
Back
Top