Controlling the Aircraft in Turbulence
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Joined: Feb 2000
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From: dunnunda
popay
Old Smokey, said in part,
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.
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
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.
Joined: Feb 2004
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From: United Kingdom
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
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
Joined: Jun 2004
Posts: 1,843
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From: Australia
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, 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
Joined: Apr 2004
Posts: 467
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From: Europe
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.
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.
