The Coffin Corner

On relating to the previous topic, we talked about how the true airspeed on an aircraft increases with altitude to a point where the airplane will climb no longer on the indicated airspeed but at a certain Mach number.  At this point, the true airspeed of the aircraft in the climb will slowly decrease till the airplane reaches the tropopause and then stabilize at a certain speed. 

The indicated airspeed was said to also be decreasing but will not stabilize as the airplane continues to climb at a constant mach number.  The graph to the left graphically displays what I am trying to write textually. 

As the airplane slows down to the V_S1 speed at this high altitude, the same thing will happen at the lower altitude; the airplane will stall at its given stall speed accounting for the non-standard temperature and non-standard pressure.  As the aircraft climbs, the stall speed will slowly increase

It would make sense that as the airplane increases in altitude, the indicated airspeed will decrease to a point where the airplane stalls.  At this altitude, the two options are to increase the indicated airspeed by increasing the Mach number or to descend.  If we were to increase our Mach speed there would be a speed at which we could not go any faster by design limitations called M_MO speed or max operating Mach speed.  There are very good reasons why an airplane would not want to exceed this limitation and those reasons may be covered in another post. 

So, if we cannot speed up and we cannot slow down due to the aircraft just above the stall speed we are forced to descend at a speed between the slow speed buffet (stall) and high speed buffet (just above the M_MO speed).  The range of speeds is called the coffin corner and for good reason.  If the airplane climbs high enough, it is in both a stalled condition and a high speed buffet.  Needless to say, the increased induced drag from the stalled condition and the high speed effects of the compressed air can create stresses on the airplane that will far exceed the design limits and tear it apart.

Notice this PFD from an unknown aircraft on the right.  We can see quite clearly the high speed MMO speed tape (top red of the airspeed tape) at FL340 and the low speed awareness tape (bottom yellow on the airspeed tape).  As this aircraft approaches FL400 and higher, the high speed tape will lower and at the same time the low speed tape will increase.

A graph of this phenomenon is also located on that website.