Flying Questions

[Babaleka]

I have scoured the Internet and my aviation books for answers to the following questions but have been unable to get an explanation that is simple enough for me to understand .Can someone please explain ?

1. Looking a a picture of a large aircraft in flight the nose appears to be raised. To my mind this indicates that it is climbing or rather should be but it is not. Is this the "angle of attack ?" How can an aircraft be pointing upwards and not climb ?

2. I understand that for an aircraft to turn the primary componants are the ailerons - "one goes up and one goes down". That makes sense. While coming in to land at Heathrow on a 757 the aircraft was doing some pronounced manouvering but much as I watched the wing (I had a seat opposite the wing) I could not see any of the surfaces move. The same has happened on other flights. What then is causing the aircraft to turn?

3. I do a fair amount of flying as an SLF and by my reckoning most decents on medium haul flights start anything from 20-25 minutes before touchdown. Is this standard ?

4. Is it a generally accepted policy that after take off SLF can move to vacant seats of their choice - after requesting to do so from a member of the CC ?

I almost feel too embarrased to ask the above and I have a few others but these will do for now.

Thanks in advance

[Hans K.]

1. Looking a a picture of a large aircraft in flight the nose appears to be raised. To my mind this indicates that it is climbing or rather should be but it is not. Is this the "angle of attack ?" How can an aircraft be pointing upwards and not climb ?

2. I understand that for an aircraft to turn the primary components are the ailerons - "one goes up and one goes down". That makes sense. While coming in to land at Heathrow on a 757 the aircraft was doing some pronounced manouvering but much as I watched the wing (I had a seat opposite the wing) I could not see any of the surfaces move. The same has happened on other flights. What then is causing the aircraft to turn?

***********************************
1. As my instructor would shout " pitch power trim " . High angles of attack are used at low speeds to increase lift , with flaps and slats adding smooth airflow over the wing . Small winged jets fly by the angle of attack indicator
a few degrees too much , things go bad quickly .

2. At higher speeds ( clean ) no gear -flaps -slats out they use inboard ailerons
near the wing root . The roll rates / inputs are too quick if flown with outboard surfaces at speed . The rudder can roll an aircraft as well depending on the aircrafts surfaces .

[BOAC]

Don't be embarrassed - you chose the right forum

1. Looking a a picture of a large aircraft in flight the nose appears to be raised. ....... How can an aircraft be pointing upwards and not climb ?

Many airliners fly at a 'deck angle' of around 4 degrees. All decided by various design considerations of undercarriage length and other factors which decide at what angle the wing is attached to the fuselage etc, and at that 'deck angle' the lift generated by the wings is equal to the weight hence no climb.

2. I could not see any of the surfaces move. The same has happened on other flights. What then is causing the aircraft to turn?

They were almost certainly moving but do not need to move much to bank the aircraft.

3. 20-25 minutes before touchdown. Is this standard ?

Pretty much so - I was always taught to start descent around 20 minutes before landing in swept wing transports and it worked!

4. Is it a generally accepted policy

No, not 'generally accepted' but by no means rare - as you say, c/crew decide. Back in allocated seats for landing.

[windypops]

Actually I think they are excellent questions to ask.

1) BOAC has already answered this but all I would add is next time you get an opportunity to go to an airshow, look out for a slow speed pass by a fighter jet. eg if you get to see the F16 display they do a slow speed pass which is level flight with a huge nose up attitude. It's not climbing as at that moment in time the lift being produced is equal to the weight which will equal "level flight". You'll even see this at other air shows with airliners.

2) as has already been stated, on larger aircraft the outboard control surfaces are locked during certain phases of flight and they use inboard ailerons. On our King Air's (which do not have inboard ailerons), you have to look very closely to see any movement of the ailerons at everything but slow flight. The inputs required at higher speeds is very small.

3) 20-25 minutes to descend is about right yes. Usual decent rates are 1000 - 1500 fpm (feet per minute) and taking a very basic example to keep things simple, lets say you were crusing at 30,000ft and your destination airfield was at sea level, to get from the cruise to airfield elevation is a simple sum. At 1,000fpm this would take 30 minutes and at 1,500fpm this would take 20 minutes. We normally plan for a 1,200fpm descent rate for pax comfort so any value between 20 and 30 minutes sounds about right.

Looking forward to your other questions

Windy Pops

[SNS3Guppy]

1. Looking a a picture of a large aircraft in flight the nose appears to be raised. To my mind this indicates that it is climbing or rather should be but it is not. Is this the "angle of attack ?" How can an aircraft be pointing upwards and not climb ?

Babaleka, the nose of the airplane pointed up is the pitch angle. Angle of attack is something you can't really see; it's the angle between the "relative wind" (slipstream) and an imaginary line drawn from the front of the wing to the back (the "chord line").

Angle of attack can vary in cruse, depending on weight and speed.

A positive pitch angle doesn't necessarily mean a climb. Think about watching a boat in the water. Think about the angle the boat makes; sometimes fairly level, but often with the bow slightly elevated.

If the airplane's pitch doesn't change at all, but power is increased, then the airplane will climb. Climb performance is a function of excess thrust (power). If the airplane is flying a particular angle and speed, increasing power will result in a climb, and decreasing power will result in a descent. In cruise, with power set, there's just enough thrust to keep the airplane at it's present speed, at this particular angle. Increase thrust and decrease the angle, one may remain at the same altitude, but speed up. Decrease power and attempt to remain at the same altitude, one must increase the angle to compensate.

2. I understand that for an aircraft to turn the primary componants are the ailerons - "one goes up and one goes down". That makes sense. While coming in to land at Heathrow on a 757 the aircraft was doing some pronounced manouvering but much as I watched the wing (I had a seat opposite the wing) I could not see any of the surfaces move. The same has happened on other flights. What then is causing the aircraft to turn?

If the airplane was being made to bank, then some form of roll control was in use...meaning something is moving. Both inboard and outboard ailerons are available; the inboard being used primarily during high speed flight. Spoilers also aid in roll-control. Control surfaces are rather large; not a lot of deflection is required to create movement about the long axis of the airplane (roll); you may not see much movement out of the control surfaces, at all. Once a turn as been entered, the controls may be neutral and not hardly moving. The slower the airplane, and the faster the roll desired, the more movement you'll note on the controls.

3. I do a fair amount of flying as an SLF and by my reckoning most decents on medium haul flights start anything from 20-25 minutes before touchdown. Is this standard ?

Somewhat. Some arrival procedures have "step-down" altitudes or published altitudes that a pilot must use on arrival. Other situations require that the pilot simply comply with altitudes provided by air traffic control. Generally speaking, one descends on a 3:1 ratio. That is, for every thousand feet of descent required, one starts three miles out. A thirty thousand foot descent is generally planned as starting at least 90 miles out. I generally add ten or twenty miles to account for leveling off and slowing down as required.

Many aircraft today have computer functions on board which will calculate the descent point for the pilot, or allow the pilot to put information into the computer which allows calculation of the descent using certain factors. A controller may direct that the flight descent to reach a particular altitude at a particular distance before a fix, for example. One can enter the fix into the computer, enter the desired altitude, and let the airplane figure it out. Or one can calculate it mentally and do the descent.

If one is flying at 35,000' and is required to descent to 25,000' twenty five miles before reaching a fix, for example, one will want to determine how fast one is going, and the distance-time question. At 480 knots for example, one is traveling 8 miles per minute. If one has to lose ten thousand feet, a 3:1 descent means starting out 55 miles from the fix (25 is the point where we need to be level, plus 30 miles for the descent). 30 miles at 8 miles a minute is about four and a half minutes. Four and a half minutes to descent ten thousand feet means about two thousand feet per minute will be required. Two thousand feet per minute will actually get us down a bit early, which is just fine.

Other methods involve constant airspeeds or constant descent rates, with other factors being varied. At any rate, if you figure a 3:1 descent from 35,000', then the descent will commence at 105 miles. At a closing rate of 8 miles a minute, that's about fourteen minutes. Because arrivals also involve maneuvering and routing, and one has to be slower for the last part of the descent (250 knots below 10,000'), then it's going to take longer than fifteen minutes. Planning the descent 20 to 30 minutes out is realistic.

Some places typically want the airplane down early, some very late, and most just about average.

4. Is it a generally accepted policy that after take off SLF can move to vacant seats of their choice - after requesting to do so from a member of the CC ?

Most places it's not a big deal, but of course, always do so with the blessing of the cabin crew.

Most airline operations use standard weights for passengers, so calculations involving weight and balance of the airplane don't change appreciably when you move to another seat...unless the airplane is half-full or half-empty and everyone gets up and moves to the far end of the airplane at one time.

Don't feel shy about asking questions. You're encouraged to do so. Don't be embarrassed. Far better to learn, than to wonder in frustration!

[Tasslehoff]

Does the rudder not get used for banking the aircraft in some instances, hence no (or very little) movement is seen on the Ailerons?

[SNS3Guppy]

The rudder gets used very little. It serves as a "turn coordinating" device, rather than a turning device. While it's true that depressing the rudder will cause a yaw, which will cause a change in angle of attack and ultimately a turn, it's not the way turns are made.

Turns are made by banking the wing, and then the rudder in some cases may e used to "clean up" the turn by slightly adjusting the airplane about the yaw axis for imperfections in the turn. One of the chief "imperfections" in the turn would be adverse yaw...a product of the ailerons and spoilers.

In many turbojet aircraft, the rudder is hardly touched, save for landing and certain situations such as an engine failure.

Some large airplane don't even connect the autopilot to the rudder except for turn coordination, during the final approach with flaps extended, and configured to land.

[Babaleka]

To all those that responded. Thanks so much for your comprehensive answers - such is the joy of being a member of PPrune.

As I thought they might , the answers raise a couple of questions and at the risk of going off thread ..................

In the climb and while banking the angles seem acute at least to an SLF. I stand corrected but I recall hearing on Bruce Dickinsons Heavy Metal him saying the pilots were not allowed to climb at an angle of more than 13 degrees . Does that apply (under normal circumstances) to the decent ?Banking angles seem way more than 13 degrees.

Another question is about how much turbulance an aircraft can deal with. It was explained to me once (by a Flight Engineer on 747's) that if a 747 was dropped from 50 feet it would not sustain any damage and that gave me huge peace of mind , particularly when the aircraft has had a heavy landing.

also

A question I have always wanted to ask : We talk about "having had a good or bad flight" which can depend on many things . But what about in the cockpit ? As a general comment , what makes for a good pilot? Is it flying the aircraft in such a way that the flight is smooth or perhaps his /her ability to stick to time schedules, good communications and so on. Granted it could make for a huge topic but it's just a question.

Thanks in advance for the responses.

Babaleka

[BOAC]

an angle of more than 13 degrees

- that is incorrect. If there is a 'normal' limit to pitch attitudes it is generally 'advised' to be around 20 degrees, but there is not really any reason why this cannot be exceeded. Generally 20 is reckoned to be the 'maximum' for passenger acceptance. Bank is normally limied to 30 for the same reasons and because all manoeuvre margins are calculated at that angle.

[SNS3Guppy]

Another question is about how much turbulance an aircraft can deal with. It was explained to me once (by a Flight Engineer on 747's) that if a 747 was dropped from 50 feet it would not sustain any damage and that gave me huge peace of mind , particularly when the aircraft has had a heavy landing.

The B747 has a maximum rate of descent for landing at max landing weight of 600 feet per minute, and at max takeoff weight of 300 feet per minute. A drop from 50' is equivalent to a 3000 foot per minute touchdown; definitely adequate to damage the airplane. I believe the FE was selling you something that perhaps he ought not.

The amount of turbulence an aircraft can handle really depends on the aircraft, how it's loaded, and it's speed at the time. The nature of the vertical gusts and their intensity directly impact the load on the airframe. Aircraft can, and do break up in turbulence and can and do experience losses of control in turbulence. The degree of turbulence required to cause an upset or structural damage, however, is substantial, and very uncommon.

When entering turbulent conditions we have maximum speeds to fly, which are directly related to protecting the airframe from excessive loads. We also minimize abrupt control inputs, particularly in turbulent conditions.

[glhcarl]

1. Looking a a picture of a large aircraft in flight the nose appears to be raised. To my mind this indicates that it is climbing or rather should be but it is not. Is this the "angle of attack ?" How can an aircraft be pointing upwards and not climb ?

It is not the "angle of attack" you are seeing but the "angle of incidence"

Using a side view of an airliner draw a line done the side of the fuselages at the window line. Now draw a line from the center of the wing leading to the center of the wing trailing edge. The angle between those to lines is the "angle of incidence". In stright and level flight the airliner will fly nose
up at approximately the same angle.

[SNS3Guppy]

While angle of incidence is the angle between the chord line of the wing and the longitudinal axis of the aircraft.

It has no correlation with pitch angle (sometimes called deck angle).

[ionutzdrg]

hello everybody, i had a disscution with a friend from school about : what happens if a runway has its left side more slippery than the right one.
Is the plane going to turn to the left so fast that the pilot does not have enough time to react and a sistem on the plane corrects the reverse thrust so that the plane continues going strait ( like ABS on cars ) or
thye plane is going to slowly head to the left side of the runway and the pilot simply corrects that using the pedals?

[Atreyu]

If the runway had one side which was so slippery as to make the aircraft uncontrollable, the crew simply would not take off or land from that runway. This would be defined as a contaminated runway, and there are huge restrictions on operating from such surfaces. I can't think of any situation in an airfield environment that would cause such a huge difference in friction left or right of a runway centreline, but I stand to be corrected.

When a runway has something other than a reasonable amount of liquid water on it, the airfield operations department will send a vehicle onto the runway to make an assessment of the runway condition, using a friction measurement device known as a Mu meter. This data will help the airfield authorities compile a code, called a Runway State Group, listing the Runway designator, what kind of contaminant is on the runway (i.e. snow, frost, ice etc), the extent of this contamination (in a percentage of total length), the depth of the contaminant in mm, and a two digit code telling us what Braking Co-efficient or Braking Action to expect. (Caution should be used when considering braking action however, the friction measuring equipment sometimes can get it quite wrong)

For example 88620494 would mean All Runways/Slush/11%-25% Coverage/04mm in depth/Braking action Medium/Good.

Using this code and our manuals in the flight deck, we can determine whether we would be approved to operate from a runway in this condition.

To answer the first part of your question about a runway, half of which is slippery, the braking action would be reported as 91, or Poor, which means no crew would be landing or taking off

Secondly, yes aircraft do have anti skid, in fact the ABS in a car was developed from the first aircraft anti skid systems.

However the selection of reverse thrust is completely independent of any anti skid system, and can only be selected by the pilots.

Correction of any skidding on the ground in an aircraft would involve releasing pressure from the brake pedals, regaining directional control using the rudder pedals and the nose gear steering, then reapplying the brakes to slow down, I don't remember being taught to use asymmetric reverse thrust, but I suppose it's always an option :P

Atreyu

[ionutzdrg]

Thanks for the reply, i really apreciate