點解要知道咩係Mach Number 呢?
因為平時low speed flight (subsonic flight) 我地假設咗空氣係 incompressible，原因係空氣有足夠嘅時間收到pressure warning ，所以啲空氣撞到飛機之前已經upwash 咗~
但實際上空氣愈接近 supersonic 嘅時候空氣就會開始 compress，因為飛機同 pressure wave 嘅速度開始接近，所以pressure warning 嘅distance 愈嚟愈少，直到飛機嘅速度等於 pressure wave 嘅速度， 即係Mach 1，就會形成bow wave，然後air density & air pressure 會同時增加~
**Pressure wave travel at speed of sound all the time**
Mach Number 係指 TAS (local speed) 同 local speed of sound 嘅比例，而Mach Number 數學上嘅公式係: M = V/a
M = Mach Number V = TAS
a = local speed of sound = 38.94 x square root Kelvin Temperature
M = 0.7 = 70% speed of sound
M = 1 = 100% speed of sound (sonic speed)
M = 2 = 200% speed of sound (2 times the local speed of sound)
Local speed of sound 會隨住溫度改變，溫度愈高 local speed of sound 愈快; 溫度愈低 local speed of sound 愈慢。
(↑Temp, local speed of sound ↓)
(↓Temp, local speed of sound ↑)
如果一架飛機以 Constant IAS climb 嘅情況之下，TAS 會隨住flight level 上升而增加， 而local speed of sound 會隨住flight level 上升愈黎愈低，所以 Mach Number 會愈黎愈大
(↑M = ↑V/a↓)
(Constant IAS, ↑Alt, ↑TAS)
而一架飛機以 Constant TAS climb 嘅情況之下，local speed of sound 會愈黎愈低，所以Mach Number 會愈黎愈大，直到過左tropopause Mach Number 就會remain constant
(i.e., local speed of sound "a" remains constant at tropopause)
(↑M = V/a↓)
(M = V/a at tropopause)
而實際上飛機有Vmo & Mmo 嘅限制，所以唔可以只係依賴一個speed reference 去飛。一直跟住IAS climb 就會過左Mmo; 相反一直跟住Mach Number descent 就會過左Vmo~