Technical Questions

[KelvinD]

I hope nobody minds but I want to ask a couple of technical questions, relating to aircraft. I know! Isn't that silly?
Anyway, I was wondering about the differences between Airbus & Boeing aircraft (besides the different spellings!). I have noticed that while Airbus aircraft on approach seem to have a definite nose up attitude, Boeing aircraft appear to be level during the same phase of the flight. As far as I can see, without resorting to a 500 foot spirit level, this is consistent one arrival after another. So, what is it that makes one raise the nose while the other, at presumably very similar speeds, rate of descent etc, keep it level?
Next question, also relates to Airbus aircraft on approach, specifically the A380. I understand the principle of using the throttle to modify the rate of descent and one can often hear engine power increasing at times, presumably to decrease the rate of descent. However, almost invariably, an approaching A380 seems to have the throttles governed by some musical maestro and engine tones can be heard quite clearly being modulated with a sort of "up a bit, down a bit, up a bit..." rhythm. This is regardless of operator and seems to begin at roughly the same distance from the airport. So, what is going on there?
Finally, wake vortices. All aircraft will drag the air behind it, with differing effects on the ground. I first noticed this while standing in a field almost directly below a landing aircraft and the sound that followed the aircraft, I likened to someone ripping up a huge sheet of paper. Over the last few years, I have listened to many and varied landing aircraft and noticed some marked differences between them. I thought it may be due to different models having different shapes etc but found by watching 767s of different operators with both identical and different models that there seems to be no consistency. Some are rather loud while others may be muted, te they were all arriving at roughly the same speed, altitude etc.. One day, while watching aeroplanes at Heathrow, in company with a BA cabin staff enthusiast (with a 20 year engineering background), when a Kuwait 777 arrived. It was relatively quiet until a good 30 or more seconds after it passed overhead, there was an almighty bang which made us both jump, followed by the paper tearing sounds. So, I wondered why every aircraft has a different noise profile with different sound levels, different delays etc?

[Johnny F@rt Pants]

Point 1 - you’re eyes are deceiving you, now I only fly one particular brand of the Boeing family, but I can guarantee you we fly the approach with a nose up attitude primarily. It depends on the flap setting we have chosen, if we land with full flap then we are almost dead level on the approach. Basically it’s all to do with aerodynamics, wing design and high lift devices.

Point 2 - thrust is used in the approach to adjust speed, not rate of descent, we change the pitch attitude to do that, then adjust the thrust accordingly to keep a constant speed.

Point 3 - no idea.

[Uplinker]

I should be doing some (boring) paperwork, so only have time for fairly general answers. My comparisons are between the Airbus 320/321/330 (tel:320/321/330) and the Boeing 737 -300/400 (tel:737 -300/400) ‘Classic’, all of which I have flown.

1) The rigged angle of the wing in relation to the fuselage might not be the same for aircraft from different manufacturers. Boeings have many more flap angle positions than an Airbus: 7 for a Boeing 737, 4 for an Airbus 320 (tel:318/319/320)/321/330.
Airbus almost always land with flap 4, (unless it is very gusty), Boeings can chose flap 30 or 40 for normal landings.

2) The Airbus autothrust is very good, the Boeing (737 classic) one is not so good. The Airbus fly-by-wire compensates for the pitch-power couple caused by power changes, owing to the underslung engines, the Boeing 737 classic does not. So manufacturer’s advice is for Boeing pilots to use manual thrust if flying manually; Airbus pilots can use autothrust when manually flying. Therefore, you will most likely hear the Airbus autothrust and the Boeing manual thrust on final approach. Obviously changes of thrust in reaction to speed changes will not be the same from the computer versus the human.

Airbus has a different philosophy to final approach speeds than Boeing. Airbus sets a constant groundspeed, Boeing sets a constant airspeed. So wind speed changes affect the two aircraft type’s thrust differently.

So there will be differences between Airbus and Boeing thrust depending on wind speed conditions and whether the aircraft are being flown manually or not at the point where you see/hear them.

3). Aircraft noise is caused by many things. Different engines, different wings, (winglets or sharklets or none), flap setting, whether various noise reducing devices have been fitted; e.g. tiny fairings in front of the fuel tank vents to reduce the ‘blowing across a bottle top noise’. Flap type and fairings. Whether the gear doors are re-retracted flush once the gear is down, (Airbus), or left open, (Boeing Classic). The landing weight of the aircraft on the day will require different engine thrust on approach, etc, etc.

Hope this helps !

[paulc]

Kelvin - I was at Heathrow a few months ago and saw a 777 wing tip vortex manage to dislodge some tiles on a house a few seconds after it went by.

[eckhard]

My experience, for what it’s worth, for normal approaches, i.e. not involving flap/slat problems or any other requirement for an increased speed.

Boeing approach attitude (737, 747, 787): between +1 and +3 degrees, depending on flap setting and wind correction.

Airbus approach attitude (319, 320, 321): between +2 and +5 degrees, dependant as above.

So yes, I think that one could say generally that Airbuses approach at a slightly higher attitude.

[DaveReidUK]

The rigged angle of the wing in relation to the fuselage might not be the same for aircraft from different manufacturers.

I believe the wing incidence on the 737 at the root (which is where it's conventionally measured) is 1°. I haven't been able to unearth a corresponding figure for the A320.

[KelvinD]

Many thanks to all for the responses.
It is nice to note that, even at my age, I can still be educated!
JohnnyF@rtPants: The idea of using thrust to modify rate of descent was told to me 40 years ago by a former US Air Force pilot who argued that more thrust equals faster air flow over the wing, equals more lift. Whereas merely pitching the nose up could you at risk of a stall. Nevertheless, I shall take your word for it now as you are doing it for a job! The bloke who informed me was flying Boeings a wee while before you, but they would have been B29s! He was an interesting, larger than life, character who has recently died; Fred Koval of Gulfport. Google him and you will see what I mean by "bit of a character"!

[Groundloop]

The idea of using thrust to modify rate of descent was told to me 40 years ago by a former US Air Force pilot who argued that more thrust equals faster air flow over the wing, equals more lift.
The idea thrust=rate of descent, pitch=speed was how I was taught in my PPL many many years ago. However, at least for for airline flying, it is taught the other way round as described above. Does anyone know if any PPL schools still teach the old way?

[Discorde]

The debate on power vs pitch for control of approach path and speed has been occupying the brains of experts over many decades. The issue centres on defined vertical flight paths. In level flight (a defined flight path) airspeed is controlled by thrust and flight path by pitch adjustment. In climbing flight the opposite applies because thrust is non-varying and so airspeed is controlled by by pitch adjustment.

There is a difference between 'descending flight' and 'approach'. In the former, in which the vertical flight path is not defined, airspeed is controlled by pitch adjustment and rate of descent (and hence gradient) by thrust. But on the approach the flight path is defined - a steady path at a constant angle (such as an ILS glideslope) and so it can be argued that airspeed should be controlled by thrust and flight path by pitch adjustment. Of course the desired end product is the same whichever control method is used.

[Discorde]

Other circumstances being equal, aircraft with leading edge high-lift devices (slats or droops) extended will invariably fly with higher attitudes than those without because of the increased leading edge camber, the higher attitude restoring the wing's effective angle of attack. Prop-driven aircraft (and first generation jet aircraft) without these devices approach with lower pitch attitudes. Compare images of early B707s and DC8s against today's types.