Calculating slope takeoff distance for C172
 

Hi everyone,
I have a question. I'm flying the Cessna 172. I wanted to know if there is a formula to calculate how much distance is needed to add or subtract regarding runway slope. I know how to obtain the runway slope gradient. I just need to know how.
For example, I was fooling around with Jeppesen FliteMap which has a takeoff distance calculator. KLVK (400 ft), Runway 25R (5253 feet) has a downslope of 0.5% which Jeppesen say it decrease the takeoff distance by 43 feet, assuming the temp in KLVK is 14C and winds calm. how did Jeppesen calculate that?

Information for KLVK

Density altitude of 400 feet
has added 78 to the standard takeoff distance.

Runway 25R (5253 feet) takeoff distance is 1665 feet.
0.5% downslope has decreased your
takeoff distance by 43 feet.

Page 8 of CAA Safety Sense Leaflet 7 contains reasonable approximations. For some reason FAA has never worried much about any performance figures not in the POH, whilst the UK's CAA has always taken it a lot more seriously. Perhaps due to the much shorter runways we tend to operate from in the UK.

https://publicapps.caa.co.uk/docs/33/20130121SSL07.pdf

G

I have always told my students in the event of lack of POH data to use a 10% per degree of slope correction factor for the takeoff distance to 50 feet numbers for a downslope and 20 % per degree for an upslope

I got this from a literature review where the 10% number seemed to be the most common correction to factor takeoff distances by authors who examined this question.

It is unfortunate that many GA airplane manuals don’t provide this information for each model.

Make sure the aircraft`s tyres are at the correct pressure for the weight of the aircraft,especially on wet/soggy /softish ground...
and make sure you use full-length of the runway..
and apply `full power` before letting the brakes off..
and...?

Thanks for the PDF file link. I think that is useful for what I'm trying to do. I think I have to agree with you regarding the FAA because here in the USA, most airfield have runways longer than say 3,000 ft and the slope gradient for the runway is not above the 1% or at the majority of them anyway. A runway slope of 0.5% (up or down) probably doesn't have much effect on the takeoff distance and in their view, the FAA probably hasn't had to worry much about it. Nevertheless, I think its still important to calculate. Upon reviewing the leaflet, I see that for a wet runway, the takeoff distance doesn't have a factor yet the landing does (10%, 1.10) Am I to presume that the same landing factor can be use for takeoff as well?

Thanks again.

If the purpose of the calculation is the exercise itself, then you work with the airplane manufacturer's data. For a larger airplane, the data will be in the flight manual, because it matters. It's not there for a 172, and not even for the Caravan. For a 172, I opine that there is more variability in performance result because of pilot technique variables rather than the runway. Of course, you can find a runway which is outside what Cessna published for performance. Cessna is not giving you data for that, so, should you choose to operate from that runway, you're on your own. If Cessna provides data, they become responsible (and liable). There is no requirement that they provide the data, so why would they accept any more responsibility and liability that necessary?

As I have done in such situations, if I need to operate from a runway with unusual characteristics, I go there first either with a locally familiar pilot for their wisdom, or solo and light. I have the wind favouring what I'm going to do, and practice. If 100 feet of ground roll is a make or break for safety, I should not be there - either ever, or under those conditions. A few times, I have waited either for cooler weather, or more favourable winds, or not taken the load.

The increase on wet runways for landing accounts for the reduced braking possible. The normal schedules assume reasonable braking is possible. The landing distance factoring for wet grass runways can show a big increase. See the table within the CAA 'Safety Sense Leaflet: Aeroplane Performance' because this gives a factor of + 60% on short wet grass!