點樣知機場QNH? 點樣知Local Pressure? How to know the QNH? How to know local pressure?

今日上完堂再唸左成個下午先好D 通左D, 所以一定要記底佢…小喇叭, 呢個Concept 好難明! Anyway, mark 低佢先!

Reading (in feet)  = (Set QNH – Actual pressure level) * 30

For example, if the aerodrome is 30 feet Elevation and 當時QNH是1013, then

Reading (in feet)  = (Set QNH – Actual pressure level) * 30

30 = ( 1013 – Actual pressure level )＊ 30

30/30 = ( 1013 – Actual pressure level )

Actual pressure level = 1013 – 1

Actual pressure level = 1012
這裡表達的是當用1013 做Reference Datum 時, 飛機就會experience 1012 pressure level, 而對應既高度就是Elevation, 即30 feet

同理, set altimeter to 0, read 到就是local experience pressure!

考試題目:

由於QNH 會變, 如果知道機場既Elevation, 我地就可以推出當時既QNH!

For example, 機場ELEV 90 feet

1. Set 到 read 到 30 feet, 取得QNH, 假如是1015
2. then use formula QNH – 1 得到 X (actual pressure level),
   30 = (1015 – actual pressure level) * 30
   30/30 = 1015 – actual pressure level
   1 = 1015 – actual pressure level
   actual pressure level = 1015-1 = 1014 hPa
   那代表飛機當時Actual pressure level 是1015-1 = 1014 hPa
3. then use formula again, Reading (in feet)  = (Set QNH – Actual pressure level) * 30
   ELEV = (QNH – X ) * 30 
   90 = (QNH – 1014) * 30
   90/30 = QNH – 1014
   QNH = 3 + 1014
   QNH = 1017 hPa