Due to efficiency considerations related to its bow wake, the supersonic transport aircraft must maintain a cruising speed that is a constant ratio to the speed of sound (a constant Mach number).
If the aircraft flies from warm air into colder air, should it increase or decrease its speed? Explain.

decreases.

Explanation:

When the aircraft is flies from the warm air into the  colder air then its speed will be decreases.

as we know that

we know mach number is constant  

so that here Mach number M is expressed as  

M =      .............................1

here u is  Local flow velocity with respect to the boundarie and v is the speed of sound in the medium

If the aircraft flies from hot air to cold air, the speed of sound in the medium will decrease. But the Mach number remains constant. Therefore, the local flow velocity relative to the boundaries also decreases.