Reference Turbine Pilot's Flight Manual Chapter 15
The SPEED OF SOUND in the atmosphere is solely dependent upon air temperature.
Temp (K) = Temp (C) + 273
Speed of sound (kts) = 39 x √ Temp (K)
A MACH METER is an instrument containing an airspeed capsule and an altitude capsule, which calculates the ratio between the aircraft's TAS and the local speed of sound. The same Mach number can occur at markedly different indicated air speeds.
Airflow can be classified as SUBSONIC, TRANSONIC, or SUPERSONIC, dependent on how it's speed compares to the speed of sound. Transonic flight usually ranges from 0.75 to 1.20 M. Within this regime there is both subsonic and supersonic airflow.
CRITICAL MACH NUMBER is the highest flight speed possible without supersonic airflow over any part of the aircraft.
Air is compressible. When an aircraft flies at slow airspeeds, the surrounding air molecules have time to 'get out of the way'. As the vehicle's speed increases, air molecules begin to 'pile up' ahead of the aircraft, increasing air pressure, density, and temperature in that region... a powerful compression wave, or SHOCK WAVE, forms at the boundary between the disturbed and undisturbed air.
I found this helpful diagram on the Wikipedia page about Compressible Flow.
As airflow passes through a shock wave:
- pressure increases
- temperature increases
- velocity decreases
LIMITING MACH NUMBER is the maximum operating speed of an aircraft in relation to the speed of sound.
Formation of a shock wave on the upper surface of a wing results in the center of pressure moving backward causing a pitch down. This can lead to an unstable condition called MACH TUCK at speeds higher than Mmo.
In high speed aircraft, SWEEPBACK design is utilized to reduce drag and increase the Critical Mach Number. An undesirable effect of sweepback design in a jet aircraft is "Dutch Roll," which is the tendency of an aircraft to roll whenever it yaws. Key characteristics of a swept wing design:
- high speed operations
- poor slow speed handling
- long range performance
- increased lateral stability
I remember back when I first started flying the Toronto - Montreal route, we were on the descent into Montreal and ATC was asking us to keep the speed up. And while we could give them about 280 kts for a while, it was also our responsibility to "not get a speeding ticket," as Captain K reminded me.
CAR 602.32 Airspeed Limitations
(a) max 250 KIAS below 10 000 ft
(b) max 200 KIAS below 3 000 ft within 10 NM of a controlled aerodrome (unless otherwise authorized by ATC)
Of course, there are exceptions: if you have a special flight operations certificate (i.e. for an airshow); or if your minimum safe airspeed for that flight configuration is greater, the aircraft shall be operated at the minimum safe airspeed.
CAR 602.33 Supersonic Flight
Put simply, it's not allowed.
While we're here, we should cover en-route speeds as well.
AIM RAC 8.3 Mach Number / True Airspeed - Clearances and Reports
(1) Assigned Mach numbers shall be adhered to within 0.01 Mach. If unable, ATC is to be informed when the clearance is issued (this is also stated in AIM RAC 12.1)
(2) If TAS is intended to change by more than 5%, advise ATC as soon as practicable.
How about holds?
AIM RAC 10.7 Holding Procedures - Speed Limitations
(a) propeller: 175 KIAS (climbing: normal speed subject to 602.32)
(b) civil turbojet: 230 KIAS up to 14 000; 265 KIAS above 14 000 (climbing: 310 KIAS subject to 602.32)
Shuttle Climbs/Descents
AIM RAC 10.9 Shuttle Procedure
Shuttles are not the same as climbs/descents in a hold, even though the patterns are similar. There may be a limit published on the instrument procedure chart, otherwise use the normal climb/descent speed for the aircraft type and airspace classification, subject to 602.32.
And one final note...
AIM RAC 9.7.3 Speed Adjustment - Radar Controlled Aircraft
Pilots complying with a speed adjustment are expected to maintain a speed within 10 KIAS of the specified speed. The issuance of an approach clearance normally cancels a speed adjustment. Once again, all aircraft are still subject to CAR 602.32.
