I'll speak for the A320, but the same applies "mostly" to all jets.
The holding "sweet spot" is FL250, up to FL300 where above compressibility can substantially increases the fuel burn at "holding speeds" by a small percent, as we need to fly at faster air speeds to stay aloft. This usually equates to 1% more up to FL 300, 4% FL350 and 11% FL398.
Conversely, 5% increase FL200 and below.
However this doesn't always mean descent is preferable. With Anti Ice on full, the penalty is between 5% and 7%.
The goal in general is for Linear holding I.e. Holding in a straight line as every turn increases fuel burn, so selecting an altitude whereby this can be achieved for a given TAS and wind forecast is the preference.
To do this, personally, I would want a minimum holding time. the actual holding time would be the beez neez, but the absolute minimum we are going to expect, on first contact, on climb.
"Minimum holding time of 10 minutes, lower altitude available, speed reduction approved"
Just helps limit the negative consequences of the system we have to live with.
Last edited by Bula; 20th Jul 2017 at 07:53.