Not if you have to use 50% off the headwind and 150% off the tailwind
As I wrote above, most of the rules must be symmetrical around zero for small deviations. Google. That is just how the physical world works.
For large changes it breaks down, obviously. If your TAS is 100kt and you have a 90kt headwind, compared to a 90kt tailwind…
The first rule of thumb I was taught in aviation isn’t a practical one, but over the years I found it to be quite accurate:
Aviation = π
Meaning everything in aviation will cost three times more or take three times longer than expected :)
So any rule which says a 1kt headwind needs 2% less runway but a 1kt tailwind needs 4% more runway has to be wrong.
Not if you have to use 50% off the headwind and 150% off the tailwind
If landing on a hill top runway, with touchdown on the upslope, a strong headwind will cause a downdraft on approach, and huge windshear. High, fast, and power added to make the runway.
A tailwind should produce updraft, but I’ve never put myself in that situation
About 3 flight bags ago, I had this book, which was full of very useful rules of thumb. Will have to look for it again…
https://www.amazon.com/Pilots-rules-thumb-aviation-formulas/dp/0963197347
I have to confess, I could rarely remember them when I needed them, and had to look them up.
Talking about RWY length, what I found useful is the rule that 10% of change in T/O weight equals 20% in T/O run. Especially for lighter aircraft, typically ULM, leaving someone on the ground or fly with less fuel means a high percentage of weight change and thus a massive improvement in take-off performance. Useful for me because I like to go to short fields.
For small deviations, it can be worked out from simple physics.
I found some good explanations online but one has to be careful because there is a lot of BS.
The first BS test is to check that the claimed rule applies equally above and below the zero line, for small deviations. That’s variously known as “calculus of small changes”, e.g. x = tan(x). So any rule which says a 1kt headwind needs 2% less runway but a 1kt tailwind needs 4% more runway has to be wrong. I think these “rules” come out of POH numbers but POH data is generated with various certification requirements which distort it. For larger percentages the linearity will obviously break down e.g. a 10% upslope will need an infinite runway length for almost any SEP.
Some stuff here with actual numbers seems good.
Peter wrote:
A 1% runway upslope is worth 6.5% extra runway length needed.A 1% runway downslope is worth 5.7% less runway needed.
Thanks. That’s quite useful. Are the figures experience-based or derived somehow?
As rule of thumb, tailwind speed ratio to stall speed should not exceed up slope gradient… it’s one vague approximation without getting into limited excess power on takeoff or using breaks on landing
With 60-70kts stall in IFR touring machine, I would say 6-7ktsfor each 1% slope
At -10kts tailwind (2%) one likely hit the limit of tailwind operations and with +5% slope it’s “one way runway”: you always land uphill and takeoff downhill, irrespective of wind
I will start a new thread when I get a moment.
A 1% runway upslope is worth 6.5% extra runway length needed.
A 1% runway downslope is worth 5.7% less runway needed.
So if you work on 1% slope being worth 7% runway length in both cases, you are doing OK. The two values are anyway equal (calculus of small changes) for small values of slope, and wind.
I need to dig out the tailwind versus downslope equivalence but recall it is something like 7kt and 1%.
It is hard to get this out of POHs because manufacturers are required to use 50% of the wind, etc.
