How regular is the rumble?

[Northbeach]

When and why we climb to higher altitudes.

"after some amount of time I would notice the engines increasing in thrust for a while, I am assuming that as more fuel burns off the higher we want to get to get on board with the jet streams? Is this right?"




The higher up into the atmosphere one goes the less dense the air. The less dense the air the less amount of drag is produced by moving a body through that “thinner high altitude” air. At 18,000’ above sea level one will find approximately half the atmosphere density found at sea level.

Given the decrease in drag resulting from the much less dense atmosphere found at higher altitudes it becomes more economical to fly at the higher altitudes; the engines burn less fuel to move the body of the jet through the air. That aircraft’s mass is generating less drag, because of the less dense air offering less resistance to its movement.

The difference is significant. Your long haul widebody carries ample fuel to make the flight from distant Australia or Europe to LAX at the higher altitudes high 30,000’ to low 40,000s above sea level (10,000-13,000 meters). That same jet most likely could not make the identical journey at 10,000’ (3,000 +/- meters). It would run out of gas long before arriving as it couldn’t possible carry enough fuel to fly the entire distance at very low altitudes. Because flying low that aircraft, in the dense atmosphere, the jet engines burn far too much fuel to make them an economical source of power. Up high in the tropopause the high bypass turbojet fan engines are in their element being able to produce enough thrust out of the thin air to overcome drag while “sipping fuel” (relatively speaking) as they drive the jet through the atmosphere.

So you are correct. Your flight starts out at the most economical altitude for that day given the weight of the aircraft, ATC (Air Traffic Control) requirements, and weather. As your jet burns off tons of fuel it becomes lighter, making flight into higher regions of the atmosphere (greater efficiency) possible. Usually your crew wants to fly higher unless there are reasons not to do that on a given day; adverse winds, turbulence aloft, high/low speed buffet margins, MEL (mechanical status) considerations and a few others.

We have some very smart computers on board, charts the manufacturer provides us plus decades of experience that provides us with guidance as to when we should initiate a climb and what altitude will result in the greatest fuel savings for kilo/pound/ton of fuel consumed for distance traveled given the weight of the jet given the winds and temperatures at altitude on that day.

Climbing higher to ride the tailwinds of a jetstream may be part of the reason to initiate the climb, as long as all the other necessary parameters are met.

Pleasant journeys………….