Reduced Thrust vs. Contaminated Runway
 

Hi,

By Regulations (e.g JAR 25), reduced thrust method on TO (i.e Assumed Temp or Flex) is not allowed on contaminated runways.

What is the physical reason behind that?

Any idea.


Thanks in advance

My best guess is that there are no hard data relating to take-off performance on contaminated runways, and therefore the authorities wouldn't allow it legally.

Contaminated runway calculations are inherenly guesstimates, so the regulators want to ensure the best power to eliminate an errors. Contamination can have a large and uncertain drag on the take-off run. This restriction has been applied to all transport category planes I have flown, civil and military

GF

Some regulatory authorities will allow flex thrust for certain types of runway contamination, wet, for example, where freezing temperatures are not anticipated or present.

In FAA-Land there are no contaminated runways, they dont get snow or ice or any other nasty stuff, therefore aircraft are not certified for operations on contaminated runways, the aircraft FLIGHT manual only contains certified data such as reduced thrust performance.

When airlines discovered that they were operating in a world that was very different to FAA-land, they insisted that the manufacturers gave them "SOME" data for operating on contaminated runways, the manufacturers decided as they weren't required to certify contaminated data, they would use the characteristics associated with contaminated runways that were developed by NASA using a Convair 880 aircraft. They then put this data as "ADVISORY" in the aircraft OPERATIONS MANUAL, which isn't a certified document.

The risk of operating on contaminated runways therefore lies with the airline and not the manufacturer. If any airline decided that they wanted contaminated data combined with assumed/flex and they had sufficient "Purchasing clout".... they could get the manufacturer to include that data.

But considering that a contaminated runway usually isnt 100% accurately reported, would you really want to take the additional risk of using Flex temperature?

Mutt

AFAIR neither of the ref documents below answers the question directly, but there is plenty of information.
Reduced Thrust Takeoff
Takeoff / Landing on Wet, Contaminated, and Slippery Runways

Thank you all for answers.

I was considering to relate my question to the "weird" effect of contamination on V1 speed. I called it "weird" because the contamination is considered to have additional decelerative effect if decision is "Stop" at V1 or difficulty in acceleration if decision is "Continue" at V1. These are just oppsoite effects which are considered in Wet/slippery runway at which reduced thrust can be done. After all these, I thought this "weirdness" somehow may be preventing reduced thrust method to be used because of Vmcg. But now I understand no conculusion of my thoughts.

Thank you again.