The great big "ask an airline pilot" thread!

Sorry if this is a dumb newbie type question but I'm curious.

What sort of engine capacity is being used once you're leveled off after takeoff and are cruising at your destination altitude e.g. 40000ft. I suppose what I'm asking is that I guess it's pedal to the metal when you're on takoff and ascending but once you're cruising, I presume the power is backed off a bit? Are you still on e.g. 7/10's of available engine power cruising? I know this isn't a simple answer at all as headwind concerns at least might be a factor on what you're going to use.

I presume that it's not also the case that using all you're available power cruising isn't necessarilly going to get you all that much extra speed?

Wow! That much! Mind you it answers another question that was bugging me in that if, as I thought, you were using 65-70%, and able to do 450kts, then you'd have a lot more power there to bring the airframe towards it's safe operating speed but as it stands, you're almost at full capacity anyway so it's a non issue!

Thanks for the reply!

Actually at high altitudes, the aircaft's airspeed will be much lower than 450 knots. It's speed is measured with reference to the speed of sound, ie Mach.

The photo attached below shows an aircaft travelling at 0.78 Mach, with a TAS of +/- 250knots.

As you will notice on the speed tape, the differrence between the lower limit(yellow line) and the upper limit(red "barber pole") that an aircraft can safely fly at high altitude is relatively small.

The higher you go or the heavier you the smaller that gap is. Consequently, the maximum cruise altitude possible for an aircraft with a given weight is known as "coffin corner"-where only a very small gap between that yellow and red line exists.

This picture is of a relatively light 737ng aircraft where there is a good margin between upper and lower limits.

(For interest sake, that little round dot on the artifical horizon to the left is called the "Flight Path Vector". It depicts where the aircraft is going in relation to where it is pointed. Way way left in this case due to a 150kt crosswind from the right-heading 174,tracking 156.)

Hi xflyer,

Not really coffin corner in the above photo.(read again I wasnt saying it was!) Flight was months ago, but there was definitely plenty of margin there to climb to FL390. In that photo, though the cruise was at FL370, the maximum was possibly FL395.

So climbing to a higher level was possible, but not a good idea given we were in a 150kt jet stream at the time, so staying lower gave us a good speed margin in the event of encountering turbulence.

The margin can be much smaller than that.

On the 737, the first warning of an overspeed is a green trend arrow on the speed tape, then the clacker aural warning when it enters an overspeed scenairo. An overspeed can be corrected before it happens and if it happens by manually retarding the thrust levers and/or using speedbrake.

An overspeed of 15 kn or greater inside the barber pole requires an engineering inspection.

In the event of Mach buffet/Tuck on any swept wing aircraft that exceeds it's critical Mach number there is the potential for an upset with a resulting control loss, severe overspeed and structural failure.

The factors for an causing an overspeed are for the most part external including turbulence and high level shear. For example it is possible(but rare) for a 50 knot headwind to reduce to zero in seconds, which results in an autothrottle power up to maintain airspeed. Now if that wind suddenly increases to 50 knots on the nose again, no autothrottle in the world will stop a minor overspeed event. It happens, I've seen it on occasion south of the Alps and Pyrnees with a northerly jet.

tippilot wrote: »

Actually at high altitudes, the aircaft's airspeed will be much lower than 450 knots. It's speed is measured with reference to the speed of sound, ie Mach.

The photo attached below shows an aircaft travelling at 0.78 Mach, with a TAS of +/- 250knots.

Hi tippilot,

Good photo. But I think you mean the aircraft has an Indicated Airspeed of 250 kts. Its real speed through the air, the True Airspeed (TAS) is more like 450 kts.
At high altitudes the IAS reduces as the air thins out and becomes less dense. So the airspeed indicator, which is calibrated for ISA to accurately indicate airspeed at sea level, 15 deg etc. needs to have a correction applied to calculate our TAS.
Mach number is the ratio between your TAS and the local speed of sound.

In relation to the OP's question, I remember reading somewhere that a typical jet engine produces only about half its sea level maximum thrust at high altitude, say 40,000 feet, for the same engine RPM. In other words an engine that can produce 26,000 lbs of thrust at sea level when running at 100% RPM puts out about 13,000 lbs of thrust at cruise altitude for the same 100% RPM. Again, this is because the air is much thinner. However the upside of the thin air is that drag is reduced, and therefore a high TAS can be maintained.

Suggestion to mods..worth absorbing this thread into the "Great Big Ask....." thread maybe?

pclancy wrote: »

In all my years of 737 nerding i've never noticed the Flight Path Vector, cheers for that info.

It's actually an Airbus thing that Boeing added on to the NG as an afterthought! It doesn't get much use but in certain situations it can be quite handy. It can be selected on or off and is something you could easily not have seen in use. It's really an individual choice, some guys use it, many don't.

eatmyshorts wrote: »

Very rarely is it "pedal to the metal" on take-off.

I like to use this analogy:
When you are driving your car away from the traffic lights, do you rev to the red-line on the rpm's, change gear, red-line again? (Maybe if you drive a Starlet or Civic with a fat exhaust you do!! )
It's the same for aircraft. There is no point in using more thrust than needed as this increases wear on the engine and can increase the chances of an engine failure on take-off. Therefore, we reduce the amount of thrust for take-off depending on factors such as weight, runway length, runway condition, obstacles and met conditions.

That's very interesting! Speaking from the point of view of a passenger, it always does seem like the engines are being gunned to get into the air but, as you point out, it's a matter of perspective. It never occured to me that a lighter loaded aircraft will get to take off point with less revs than a heavilly loaded one. A subtler point it proves is how much pilots trust their aircraft! I've often heard that an aircraft 'wants' to fly and given the right conditions it readily will. As a newbie I think I'd gun the engines because in my head I'd think 'this thing won't get into the air unless I give it plenty of welly'. That's probably why I had the misconception that max available thrust was being used.

On the ground, maybe, you can but ask. In the air, no way.

I asked a flight attendant when we were unloading after a flight (EI) and she refused to even ask the captain, citing security regulations :rolleyes:

It's really a matter of luck but it's definitely worth asking

Does the sidestick in an Airbus work in the same way a joystick would work on a flight sim? I have watched a few videos of the pilot on the Airbus 318 and he seems to be making alot of sidestick movements, looks more like he is playing FIFA 2012!

xabi

Does the sidestick in an Airbus work in the same way a joystick would work on a flight sim? I have watched a few videos of the pilot on the Airbus 318 and he seems to be making alot of sidestick movements,

That's scarebus pilots trying to make out that their aircraft are challenging to fly whereas the computer does the "actual" flying.

Everyone knows proper pilots fly Boeing.

Plowman wrote: »

This post has been deleted.

One big difference between the two is that if you are not the pilot flying in an Airbus, you can't see what inputs the pilot flying is making. The can be a serious problem, because if the pilot flying gets disorientated and loses awareness, and then starts doing something wrong then the other pilot can't see this.

This is not the case with a yoke because both are mechanically linked and will both move at the same time.