Fuel Burn in Turbulence

[Airmann]

Having been in the situation of having to make a decision between diverting or holding over an airport and waiting for wx to clear I was wondering how accurate are fuel burn estimates when faced with significant turbulence that requires constant and quite large variations in thrust in order to maintain speed.

Should traditional numbers for fuel burn be scraped at this point. Should a margin be added, and if so how much?

Hoping that someone might have some real life experienice regarding this.

[eckhard]

How about adding 20%?

[cavuman1]

Fuel Consumption in Turbulence

Vra (rough air) is always less than standard cruise. Also called turbulence penetration speed, this velocity requires less fuel whether selected by pilot hand flying (magenta line notwithstanding) or FADEC established Vmo - maximum operating speed - as long as the aircraft is maintaining or losing altitude. Obviously, climbing to escape turbulence is another story altogether.

- Ed

[172_driver]

Vra (rough air) is always less than standard cruise. Also called turbulence penetration speed, this velocity requires less fuel whether selected by pilot hand flying (magenta line notwithstanding) or FADEC established Vmo - maximum operating speed - as long as the aircraft is maintaining or losing altitude. Obviously, climbing to escape turbulence is another story altogether.

I have no idea what you are trying to say. It doesn't address the original question if constant thrust changes alters the fuel burn. Rough air speed isn't always less than standard cruise speed. Not on a jetliner anyway.

[White Knight]

Originally Posted by cavuman1

Vra (rough air) is always less than standard cruise. Also called turbulence penetration speed

Guess I learnt something today: I've never heard of or seen Vra!!!! Anyway, the Severe Turbulence checklist on my 380 says to increase (or less likely decrease) to Mach 0.85. So actually I could be increasing speed to maintain a margin between Vls and Vne/Vmo.

The difference in fuel burn is the square root of 0....

[Airmann]

Vra or turbulence speed, if I'm not mistaken, has more to do with ensuring that structural loads are kept within limits rather than having anything to do with minimizing fuel burn.

I'm talking here more about holds in terminal airspace. Below around 18,000 ft. And at the standard hold speeds with moderate turbulence or greater.