Are airliners at 100% throttle when crusing?

[Steviesam]

Probably a dumb question, but I will ask it anyway!

When flying merrily along in a commercial airliner, what sort of throttle are they using?

Are they at 100% or a bit less?

No real reason for asking, just curious!

[Geoff Stilwell]

No they are not....once at altitude and cruising speed they back right off...bit like cruise control on a car.

[Ray Luxury-Yacht]

Nope, nowhere near. They're probably not even close to 100% on take-off either.

Without going into a huge spiel about it - take-off EPR is calculated carefully by the pilots, taking into account many things like the wind, barometric pressure (i.e. how far above sea-level the airport is) and the weight of the aircraft including passengers and baggage.

The same info is used for EPR when in the cruise.

If the pilots calculate the EPR at anything over the actual level required for take off and cruise, they get a stand-up meeting without coffee with the airline's management about the cost of fuel that they have wasted.

It really is calculated to such a critical degree...

[Tc24]

Not something I'd ever thought about, but interesting.

If not at 100% while cruising, how much faster could your average commercial plane actually fly when at vmax?

[Gaspode]

What does EPR mean? And, more interestingly, why did you think we would know?

[mrmr96]

They won't fly at vmax in order to get better fuel economy.

[Ray Luxury-Yacht]

Gaspode said:

What does EPR mean? And, more interestingly, why did you think we would know?

Oops sorry - as an enthusiastic amateur flying nerd, I posted in acronym language without thinking!

EPR = engine power ratio.

The pilots have, as part of their display, gauges which display engine power as a percentage of total available power.

So as per my post about their calculations on take-off for instance, they might decide that to obtain V1 and rotate speeds ( V1 being the speed at which they are committed to take-off given the plane's weight and the runway length, and rotate being the point at which the ground speed is high enough to pull back on the elevators and 'rotate' the plane to take off) the EPR needs to be, say, 69% or whatever it might be.

[Exige77]

Think it's Engine Pressure Ratio.

Used to measure thrust.

They calculate how much thrust they need to achieve a particular climb or cruise speed.

Ex77

[Odie]

Tc24 said:

Not something I'd ever thought about, but interesting.

If not at 100% while cruising, how much faster could your average commercial plane actually fly when at vmax?

Probably depends on airframe stresses more than anything else. Its different what the engines are capable of and what the actual aircraft can actually do.

As an example the SR-71 max speed was Mach 3.3 but the engines where probably capable of taking it to over Mach 5.

[shirt]

there's also different types of cruise. either the aircraft will maintain speed [altitude increases as weight decreases due to fuel burn] or maintain altitude [speed decreases to compensate for reduced weight/increased lift]

[davepoth]

For certification, twin engined planes have to be able to take off on one if they've already hit V1. That means they have a substantial reserve of power over and above anything except "oh st" moments.

[Thankyou4calling]

Tc24 said:

Not something I'd ever thought about, but interesting.

If not at 100% while cruising, how much faster could your average commercial plane actually fly when at vmax?

Can someone answer this? Take a 747, average cruising speed is say 580 mph. If the throttles were pushed right forward to the stops what would the speed be? Same conditions, wind, load, trajectory etc.

[Grant76]

Ray Luxury-Yacht said:

Gaspode said:

What does EPR mean? And, more interestingly, why did you think we would know?

Oops sorry - as an enthusiastic amateur flying nerd, I posted in acronym language without thinking!

EPR = engine power ratio.

The pilots have, as part of their display, gauges which display engine power as a percentage of total available power.

So as per my post about their calculations on take-off for instance, they might decide that to obtain V1 and rotate speeds ( V1 being the speed at which they are committed to take-off given the plane's weight and the runway length, and rotate being the point at which the ground speed is high enough to pull back on the elevators and 'rotate' the plane to take off) the EPR needs to be, say, 69% or whatever it might be.

Cough Pressure Cough

[Ray Luxury-Yacht]

Grant76 said:

Ray Luxury-Yacht said:

Gaspode said:

What does EPR mean? And, more interestingly, why did you think we would know?

Oops sorry - as an enthusiastic amateur flying nerd, I posted in acronym language without thinking!

EPR = engine power ratio.

The pilots have, as part of their display, gauges which display engine power as a percentage of total available power.

So as per my post about their calculations on take-off for instance, they might decide that to obtain V1 and rotate speeds ( V1 being the speed at which they are committed to take-off given the plane's weight and the runway length, and rotate being the point at which the ground speed is high enough to pull back on the elevators and 'rotate' the plane to take off) the EPR needs to be, say, 69% or whatever it might be.

Cough Pressure Cough

Sorry yes, pressure is the correct term. Some pilots say power though...

[Eric Mc]

There are aerodynamic and Mach limits as to how fast any airframe will go. These speeds are quite critical and an aeroplane can have real problems if it goes too fast.

So, at crusing altitudes, throttles are set in order to ensure critical speeds aren't exceeded AND to optimise fuel burn.

[15peter20]

The engines are designed to reach peak efficiency at higher power settings- hence why flying as high as aerodynamically practical saves fuel even though the air is thinner, assuming the wind is constant.

Airlines use a 'cost index' to determine cruise speed which balances fuel cost vs wear and tear on the airframe/engines. It may be cheaper to go fast with a higher fuel burn in order to save airframe hours until the next service.

100% throttle would overboost the engines so a Max Continuous Thrust setting is calculated. Even with this set, assuming all engines are working, you'd still have enough power to exceed the max aircraft speed at a normal cruise altitude for the weight.

edit: forgot about the wind- standard.

Edited by 15peter20 on Saturday 20th July 22:33

[Dr Doofenshmirtz]

Thankyou4calling said:

Tc24 said:

Not something I'd ever thought about, but interesting.

If not at 100% while cruising, how much faster could your average commercial plane actually fly when at vmax?

Can someone answer this? Take a 747, average cruising speed is say 580 mph. If the throttles were pushed right forward to the stops what would the speed be? Same conditions, wind, load, trajectory etc.

The plane would break apart. But at what speed that happens, I'm not sure.

[Kempus]

You' d reach the aero dynamic limits of the wing with Mach buffet or tuck and this is when things tend to be ripped off with the engines at full chat for any considerable length of time. The actual speed depends on weight, altitude, temp, density etc.

The majority of our aircraft have GE90 engines which we use N1 to calculate thrust settings. N1 is the percentage of the speed of the fan at the front which funnily enough goes on occasion to 102% under certain conditions.

But as for the original question in cruise it's about 84% N1 will give about Mach 0.84 ~8 miles a minute.

Oh and for the V1 calculation, if one engine goes pop the thrust and speeds calculated are enough that you don't need increase thrust on the temaining engine(s) to get airborne safely. Adding thrust can can make things worse resulting in loss of direction control of the aircraft.

Kempus

[15peter20]

Dr Doofenshmirtz said:

Thankyou4calling said:

Tc24 said:

Not something I'd ever thought about, but interesting.

If not at 100% while cruising, how much faster could your average commercial plane actually fly when at vmax?

Can someone answer this? Take a 747, average cruising speed is say 580 mph. If the throttles were pushed right forward to the stops what would the speed be? Same conditions, wind, load, trajectory etc.

The plane would break apart. But at what speed that happens, I'm not sure.

Eventually something structural would fail. If you're at cruise altitude, then you'd get into high speed buffet first. Air over the top surface of the wing is faster than the aircraft is moving, hence airliners have swept wings to keep as much of the wing subsonic for longer. As the shock wave moves towards the leading edge as the wing becomes increasingly supersonic it causes buffet which you'd be hard pressed to push through unless in a dive as well.
I'm sure an aerodynamicist could give you a more thorough answer but this is the basic principle.

[paolow]

perhaps hijacking the thread a little - and I apologise if so but it seems topical - I flew on a 737 from Male to Gatwick and was aft of the wing and on a window seat in the cabin. For the duration of the flight it was apparent from the noise the engines were spooling up and then down ad infinitum during the flight. I understand we were flying into a 100 knot/mph (wasnt paying that much attention) headwind - was that the airline equivalent of 'cruise control' where the airliner tried to maintain ground speed within a window and used the engines accordingly?