Computing Runway Requirements

[207WF]

Our owners’ manuals are pathetic on nonstandard takeoff performance, so I am working on a set of rules of thumb to think through before takeoff. I want something practical – simple enough to do in my head. Here is what I have now; any suggested improvements would be appreciated.

 

Start with book sea level standard conditions, landing distance distance = 967 feet (book takeoff is shorter, so this is conservative. Round to 1,000).

 

Compute the approximate density altitude: add 1250 feet to your altitude per 10 degrees C above standard temperature. Standard temperature is 15C at sea level and drops about 1C per 500 feet of altitude. (Check the figure on the dynon after it powers up.)

 

Increase the distance by 10% for each 1,000 feet of density altitude up to 8,000 and by an additional 15% for each 1,000 feet above that.

 

Decrease for weights under gross by twice the percentage under gross weight. 20% under gross (1093 pounds) reduces the distance by 40%.

 

Decrease for the headwind component by 10% for each 5 knots of headwind.

 

WF

[Jim]

I copied a Koch chart...

 

https://www.faasafety.gov/files/gslac/library/documents/2006/Oct/6847/koch_chart.gif

 

to the back of my checklist.

[Jim Meade]

Our owners’ manuals are pathetic on nonstandard takeoff performance, so I am working on a set of rules of thumb to think through before takeoff.

 

Sounds like an interesting concept. Do you have specific reasons for choosing the factors you picked, and can you share them?

[S4Flier]

Increase the distance by 10% for each 1,000 feet of density altitude up to 8,000 and by an additional 15% for each 1,000 feet above that.

 

Interesting. This rule of thumb was in the most recent Sport Pilot magazine from EAA. Sounds like a good idea.

 

Don't know if I'd worry about reducing this for weights less than gross. By assuming gross you have a built in safety factor.

 

The article, and mountain flying books I've read, also mentions that you should achieve 70% of your rotation speed by 50% of the runway. This gives you enough time to abort the take-off.

[207WF]

These are all rules of thumb and approximations from various sources, culled over the years and recently from the internet. I am interested to check how close the predictions are, by noting on preflight where I should be at 50' agl and then where I actually am at 50' agl, using the runways I encounter when traveling this summer. It will be a challenge to have the mental bandwidth to note and record the data on takeoff! WF

[Runtoeat]

I copied a Koch chart...

 

https://www.faasafet.../koch_chart.gif

 

to the back of my checklist.

 

 

Jim, good chart. I copied it and will use it. Good subject to discuss. I'm thinking that with the performance of our CT's, we'll be able to keep it simple by using "worst case" calculations and be OK for takeoffs at short runways in high dens. alt. conditions. I'll have to look closer at Jim's chart. Maybe this will provide a simple estimate that errs on the side of safety.

[Jim Meade]

The thing that bothers me about worst case scenarios is that they build in assumptions. I'd rather have "just the facts" and add a safety factor deliberately, on my own.

 

We're talking about extremes here, but I'd hate to decide that I could not get out of a strip if the forest was on fire and then decide I had to risk it without knowing the actual data. I'd rather have the actual data and know the TO was possible even though close.

 

How often have we seen people take a number and add a safety margin to it? If that number already has a safety margin, then we are needlessly complicating and confusing something.

 

To each his own. I'd not say don't do it, I'd simply say I'd rather have the exact numbers. I'd then deliberately add such safety factor as the situation called for.

[207WF]

the Koch Chart is interesting, and I have stapled it to my checklist. But, it only considers density altitude. My list considers aircraft weight and wind, so it should be more accurate in practice. WF