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.
