With modern airliners nothing particularly exciting happens at Mcrit. The vast majority of cruise flight for such aircraft occurs at speeds slightly higher than Mcrit, but lower than Mmo/Vmo. In suitable conditions the passengers may see the shock waves on the wings. But these shock waves are not intense enough to cause any serious problems.
The factor that really places a practical limit on cruising speed is the large drag increase that occurs at Mcdr (Critical Drag Rise Mach Number). This makes it uneconomical to cruise at higher speeds. Mcdr is higher than Mcrit.
When told about Mcrit, shock waves and tuck under, many students imagine that that aircraft will enter uncontrollable dives and the world will end whenever Mcrit is reached. This simply is not true.
Your comments are very confused, but I will deal with each of them in turn.
soo if we are flying at Vmo the maximum operating speed of aircraft and increase the maximum AOA the airflow wont increase up to Mcrit?
We would not be cruising at Vmo. But if we were to increase angle of attack this would slow down, decrease Mcrit and also increase the acceleration of the airflow. If we were to increase thrust to maintain our airspeed and if our initial speed were close enough to Mcrit, then yes we would eventually reach Mcrit. And the airflow over some point on the aircraft would reach Mach 1.
i was wondering that as heavy aircraft fly at its maximum cruise altitude where an aircraft reaches its maximum speed where speed cannot be increases
In normal cruise flight the aircraft is not flying at its maximum possible speed. Cruise speed will be slightly less that the speed at which high-speed buffet occurs.
if it were to happen the aircraft will reach to the mach and where there is a problem to be tuck under ,
In order to get into a tuck under situation the pilot would need to ignore the high-speed buffet and keep on accelerating. He/she would need to be a fool to do this.
as it reaches to the point where he can no longer increases the airspeed he will increases the AOA to compensate for the lift to weight ratio,
This statement makes no sense. In cruise flight the weight gradually decreases as fuel is burned. So why should the lift to weight ratio require an increase in speed or angle of attack?