Controlling the Aircraft in Turbulence

[A300Man-2005]

A few things that I've been meaning to ask for a helluva long time, but never really got round to it......


1) Why do aircraft engines seem to idle back during severe turbulence, and sound as if power is being dropped, as opposed to increased? One would imagine that it would be better to "put the foot down" and clear the weather quickly. Maybe a silly question, but just wondered why.

2) Why does turbulence seem worse when sitting at the back of the plane, as opposed to the front?

3) (Not turbulence related) When aircraft are all taxiing for take off, and there is the proverbial queue for take off at the end of the runway, who stipulates how close one aircraft should get to the next one, before deciding that it's time to hit the brakes and stop? Is it purely down to pilot experinece and judgement, or is there an automated something or other in the cockpit that warns the crew that it's time to stop, or do ATCactually keep an eye from the tower?

Cheers,

A300Man

[john_tullamarine]

While I find something of a disconnect between your questions and your profile ... the following is a brief and rather simplistic overview of the answers.

(a) severe turbulence gives rise to two concerns .. stalling and breakup due to excessive structural loads. The design standards require the OEM to provide guidance to flightcrew relating to recommended speeds in such circumstances. Engine thrust will be adjusted to whatever target is appropriate to the recommended speed.

(b) turbulence will cause time varying accelerations and pitching motions. The centre of pitching motion generally is somewhere near the forward half of the wing. As the rear seats are some distance from this region, accelerations due to the pitching motions may give rise to a perception that the turbulence is "worse" than might be thought if one is in a seat row closer to the wing.

(c) most jurisdictions regulate a minimum distance. Operators (and pilots) may well prefer a greater distance.

[tunneler]

1 - Quite the opposite im afraid, when entering turbulence the norm is to slow down to a "max turbulence pentration speed" Hitting the bumps slower reduces structural loads on the aircraft. For example A320/321 usually cruises at .78/.79 but the turb speed in the cruise is M0.76

2 - Can't offer a a magical technical explanation for that but I know its all do to with the moment arms from the CofG of the aircraft, ie the further from the centre of the aircraft you are, the more pronounced any bumps are.

3 - Judgement and experience prevail - generally speaking most people will leave a fairly decent sized gap from the aircraft in front to avoid ingesting exhaust fumes which makes the cabin stink!

Hope this helps.

[Crótalo]

One thing to note regarding the various intensities of turbulence (light, moderate, severe, extreme) --

What may feel like "severe" turbulence to someone sitting in the cabin, especially in the aft sections, as you've mentioned, may in fact only be "moderate" turbulence. There are defined parameters for each level of turbulence/chop.

Obviously, I don't know what you've experienced personally, but most people don't experience "severe" turbulence on a routine basis, as operators take all possible steps to avoid it.

Moderate turbulence encounters are much more common, (although they may feel quite "severe" from an individual/subjective viewpoint). Even with moderate turbulence, most operators will attempt to find smoother altitudes, if other altitudes are reported to be better, and if they're available. However, sometimes there is no smoother altitude, so you simply fly (at the lower turbulence penetration speed, as explained in by tunneler and john_t) until you've passed through the area.

[popay]

Dear A 300 Man, its seems like you really have a disconnect between your questions and your profile. My post is slightly off topic, but I thought you wanted to speak to me on my flight, like you have announced in the ME forum. “I have many questions for you” I would have been happy to answer your questions on board, but you never showed up. Just for you information there is no F class on flights to MNL. I think the nature of your questions is based on experiences you are doing as a cabin crew, being sometimes placed at R2 or in the middle of the airplanes or at the back.
Nevertheless I will answer your questions best I can:
1. The engines idle back while encountering the turbulences because the pilot has to fly a certain speed, usually lower than the cruising speed (A 330 cruising speed is 0.83 and turbulence speed 0.80) which provides structural relief and prevents structure from being overstressed. That’s why you hear engines idling back.
2. To spare a long technical explanation, its got something with the momentum, as john_tullamarine said, the momentum at the rear is greater than in front, because the same force*grater distance= grater momentum.
3. Agree with john_tullamarine, the general rule is one aircraft length. Its also a matter of pax comfort, to keep reasonable distance in order not to intake fumes from preceding A/C.

There is nothing wrong with being a cabin crew. If you still want to see me on board of one of QR A 330, mostly welcome.
Cheers.

[simfly]

Is there problems when using spoilers during turbulence (ie in descent) due to the aircraft experienceing that loud buffet?

[Skylark_air]

____________________________________________________
"2. To spare a long technical explanation, its got something with the momentum, as john_tullamarine said, the momentum at the rear is greater than in front, because the same force*grater distance= grater momentum. "
____________________________________________________

I remember on a certain British Oldie Aircraft the cabin sometimes traveled at a velocity faster than that of the entire aircraft, so two KIAS Readings were required.

I'm unsure as to what effect this had during tubulance, but it did mean that they could use the reversers instead of the flaps on some occasions during landing.

I think that it has recently featured on Disco. Wings: "Classic British Aircraft" series, because I saw a film crew going over it during routing training in the UK a while ago.

[The African Dude]

I remember on a certain British Oldie Aircraft the cabin sometimes traveled at a velocity faster than that of the entire aircraft, so two KIAS Readings were required.
Sorry, Skylark, I'm sure I'm being dense :} really, apologies!! But how is this possible (what have I missed!?!:confused: ) and more importantly, how much did the readings differ on average?

Sounds a bit fishy!! Like a ghost-plane or..... oh ok, I'll get me coat.

Andy:ok:

[Vee One...Rotate]

A good way of picturing why bumps may feel more pronounced the further away from the centre of the aircraft you are is to imagine the aircraft as a see-saw. The pivot point and centre of gravity is roughly in the centre of the aircraft and this position is represented by the middle of the see-saw.

When the see-saw is rocked you can see that the ends trace paths through the air which are longer and more pronounced than any other part along the length of the see-saw. The closer to the middle, the less the movement and bang at the pivot point the movement is zero. Of course turbulence induces more than a simple regular up/down see-saw motion but the principle is the same.

Moment (or "turning effect") = force * distance from pivot...so a given force applied to the airframe by turbulent air will have more of a turning effect the greater the distance from the pivot.

V1R :ok:

[Capt Claret]

Only the distance question has yet to be answered adequately. From Australian CAO (Civil Aviation Order) 20.9, para 5.1.4.

and turbine engines, in addition, shall not be operated within the appropriate distance specified below of any other aircraft, fuelling equipment or exposed public areas which lie to the rear of and within a 15 degree arc either side of the exhaust outlet axis of that engine:

Engine Type Power Condition Minimum Distance Metres


Turbo-prop At or below normal slow taxiing power 15(50 ft)
(Turbo prop) At power used to initiate movement of a stationary aircraft 23(75 ft)
Turbo-jet At or below normal slow taxiing thrust 30(100 ft)
(turbo-jet) At thrust used to initiate movement of a stationary aircraft 46(150 ft)

[West Coast]

While not sucking up the fumes of the guy ahead is likely the most common reason, its not the most important for not tailgating. I don't want to be number 30 or so at KORD on a snowy day and be butts to nuts on the guy ahead only to have him shear the deice/anti ice fluid from the wing with his engines rather than on T/O roll.
Air Florida accident comes to mind as another reason not to be excessively close.

[bealine]

....be butts to nuts on the guy ahead

Outstanding terminology old chap! I hope they use that on the "Simpsons" one day!!!
:p

[G-ANDY]

Just a quick question:

When reducing power, how long does it take to get down to turbulence speed from cruise speed, and how much do you need to throttle back? Does the aircraft / computer do this itself, or is this something the crew sort out?

Thanks

[Old Smokey]

simfly,
Is there problems when using spoilers during turbulence (ie in descent) due to the aircraft experienceing that loud buffet?
There's sure problems if you encounter a large overspeed in turbulence and DON'T use the spoilers, and whatever other means are available (like completely closing the thrust levers) to avoid the overspeed. Fairly recently I was sitting in 160 knots Tailwind which became 80 knots in a matter of seconds, fully closed thrust levers AND fully deployed spoilers even then did not prevent a Mmo exceedance by M0.026

G-ANDY,
how long does it take to get down to turbulence speed from cruise speed, and how much do you need to throttle back?
Too long (although Econ cruise these days is not too far above Turbulence Penetration Speed), in a severe encounter. Quite fast enough in Moderate turbulence when comfort is the primary consideration.
Does the aircraft / computer do this itself, or is this something the crew sort out?
The Auto Thrust responds to crew inputs, the aircraft "computer" is blisfully unaware of turbulence. It has limited authority in cruise, quite sufficient in moderate turbulence, but crew intervention definately needed in severe encounters like that described at the beginning of this post.


A300Man-2005,
who stipulates how close one aircraft should get to the next one, before deciding that it's time to hit the brakes and stop?
The rules do, as indicated by Capt Claret, but a lot of common sense is still called for beyond these statuory limits. If I'm in an F50, behind a B747 on an up-sloping Taxiway, I know he'll apply a lot more breakaway thrust than normal and stay a respectable distance behind. Even if relative size doesn't matter, in freezing, slushy, snowy conditions I don't want the dude in front blowing all that hazardous ice forming stuff all over my shiny new US$5000 de-ice just 10 minutes prior to the end of the Hold Over Time.

Best wishes,

Old Smokey

[Capt Claret]

G'day Old Smokey

Would you elaborate on "recently I was sitting in 160 knots Tailwind which became 80 knots in a matter of seconds". I'm guessing the wind went from tailwind to headwind, not just a reduction in tailwind component?

[john_tullamarine]

CC,

You don't fly high enough in the jetstreams, good friend ..

JT

[popay]

C.C. I'm guessing the wind went from tailwind to headwind, not just a reduction in tailwind component? What??????????????????????????????????
I hope this is a joke from you. If not, please collect the Nobel price award, because you would be the first person on this planet, who would revoke existing physics laws.
As you know jet streams in the atmosphere is a phenomenon, caused by differences in air pressure in different parts of the planet earth, so basically the air flows from high pressure area in to low pressure area additionally accelerated through earth rotation. I have never ever seen the air flowing from low to high, for the same reason you can’t simultaneously use high and low pressure bleed valves for supply, because the high pressure air would flow in to low pressure compressor and definitely cause the engine stall. It’s either or.
So it’s definitely tail wind component reduction, but never two winds blowing against each other.
Cheers.

[john_tullamarine]

popay,

The jetstream winds are not constant and, for a given route, say from coast to coast (Oz), one can start with not much from whatever direction, end up with lots typically from the west, and then have it die down ... all depends on the character and location of the jetstream ... best I have seen was around 240kts going our way PER-MEL .. long time ago, now.

Along the east coast ... plenty of times one will find very significant lateral wind shears.

[Old Smokey]

To all the doubters..........

Northern Pacific, in the vicinity of the Aleutian Islands, Forecast (and Actual) rapid change in direction by about 60 degrees from pure Tailwind to Tailwind/Crosswind, in the vicinity of a very active front. Well SIGMETed, the aircraft ahead and behind us got a pounding, with a JAL guy behind us getting a real pounding requiring structural check in SFO.

The wind speed stayed pretty much the same, but with a rapid change in direction. Our own FDR recorded that the change took place over a 21 second period. We were already at Turbulence Penetration speed (M0.82), and rapid idle thrust and full speed brake deployment could not stop the upwards speed trend vector, reaching M0.882. All that data from the FDR, not subjective pilot report.

JAL was grounded for 4 days, at the time of his encounter, the pilot's voice on the radio sounded like that of a chopper pilot.

Regards,

Old Smokey

[popay]

Dear john_tullamarine, totally agree with you. It all depends on the geographical location etc. etc. A sudden change in wind direction up to 70 degrees are also possible, all this combined with shift in tropopause altitudes causing sudden temperature changes can end up in significant over speed at high altitudes, but never ever it would be possible two winds blowing against each other. That’s what I meant.
Cheers.

[Capt Claret]

popay

Old Smokey, said in part, There's sure problems if you encounter a large overspeed in turbulence and DON'T use the spoilers, and whatever other means are available (like completely closing the thrust levers) to avoid the overspeed. Fairly recently I was sitting in 160 knots Tailwind which became 80 knots in a matter of seconds, fully closed thrust levers AND fully deployed spoilers even then did not prevent a Mmo exceedance by M0.026

When I read this my mind set was overspeed not turbulence and so I was trying to workout how a wind change from 160kt to 80 kt could cause an overspeed. I didn't realise he was referring to turbulence until reading the subsequent posts.

Actually, re reading the quote above from Old Smokey I still don't understand how a wind speed reduction causes an exceedence of Mmo? I can understand one might exceed Turb Penetration speed but not Mmo.

[john_tullamarine]

.. wind shear due rapid reduction in tailwind ....

[G-ANDY]

Thanks for your answers Old Smokey. I'm about to buy my ATPL theory books and am looking forward to learning about this stuff. Not looking forward to all the legal bits though!

How do you know if you're approaching turbulence? Do you see it on your wx radar, or does the a/c in front report it?

Cheers,

G-ANDY

[Old Smokey]

Capt Claret, Ground Speed was 660 Kt against TAS 500 Kt, after the wind COMPONENT dropped by 80 Kt, the aircraft, due to momentum was initially still travelling at G/S 660 with 80 knots LESS Wind Component, thus a rise of TAS by 80 Kt, with commensurate rise in IAS and Mach No.

G-ANDY, Weather radar 'sees' precipitation, and cannot predict Clear Air Turbulence (We were well above the front). In our case we had plenty of warning from the SIGMET, and more particularly, the aircraft ahead which were having a bad time, hence our being at Turbulence Penetration Speed in advance. We, in turn, told the aircraft behind, didn't help the JAL guy much though!

Regards,

Old Smokey

[G-ANDY]

Thanks Old Smokey. The only turbulence I ever encounter is either flying down wind of hills, or thermals in summer!

G-ANDY

[popay]

Capt Claret, Actually, re reading the quote above from Old Smokey I still don't understand how a wind speed reduction causes an exceedence of Mmo? I can understand one might exceed Turb Penetration speed but not Mmo.
I am afraid, but Old Smokey is right. I have experienced even approach to Mcrit over Chinas north east mountains “Altay” in clear air turbulence, where in very short time Mmo was exceeded and approaching the Mcrit the shaking started, we did the same: Power idle only, since the local sonic buffets stared already. In this case the speed brakes can’t do much, but can cause an undesirable change in aircrafts dynamic stability.
Back to your question though, well let’s imagine you would have to cross the jet stream in FL 350, flying from SE to NW at M 0,83 cruising speed.
First of all a little excursion in to meteorology topic upper winds, upper winds are determined by the thermal gradient between two columns of air with a difference in temperature causing the pressure differences at high alt. even having the same pressure at sea level. This pressure differences creates a wind parallel to the isobars.
What is the jet stream?-simply narrow bands of high speed upper thermal winds at very high alt. The official definition of the let stream is strong, narrow current on a quazi-horizontal axis wind in the upper tropopause or stratosphere characterized by strong vertical and/or lateral wind shears (CAT). Typical dimensions: 1500 NM long, 200 NM wide, 12000 ft deep.
Entering the jet the temperature rises and the up and downdraft occurs, while exiting the jet the temperature falls with the same wind shears, the last part is the most dangerous part in terms of aero dynamical affects to the A/C.
What happens to the A/C at that stage? Two things: sudden increase in angle of attack do to vertical shears (resulting in higher horizontal vector speed) and decreasing in the speed of sound do to lower temperature (resulting in higher Mach number per 5 C temperature decrement add 0,005 in Mach), both of them increasing the lift significantly. The A/C has though to maintain the assigned level and the additional lift force is compensated through a significant speed increment, causing sometimes penetrating of Mmo. The Mmo isn’t far away from cruising speed, in fact for a A 330 its 0,86- which might even be reduced through g factor down to 0,85.
So, having considered that we can say, that an decrement in 10 C OAT combined with G load >1 in CAT, the A/C maintains the assigned level, having the inertia, might very well exceed Mmo.
Now I really need a lunch.
Hope it helped.
Cheers.
:=