FMS: Top of Descent

之前有人問起個Airbus點計Top of Descent?

其實嗰條(Altitude to lose) X 3 rule of thumb係好好用; 無論飛GA or airliner 都可以apply.

A330 可以用(Altitude to lose) X 3, 但要記住加埋distance to Decelerate.

Deceleration rate during LEVEL flight 大概係10kt/NM. 20kt/NM with speedbrake.

咁我就懶d; high speed (>250kt) 我就用Altitude X4; low speed (<250kt) 就X3.

個buffer 應該夠decelerate.

以上都係mental math, 咁Airbus 個FMS究竟compute個咩TOD 俾Pilot 呢?

眾所周知, Airbus pilot喺preflight ge 時候input個 flight profile 入個MCDU; 個FM就會base on 個flight profile 同constraints, 計個 vertical profile 出黎~

*MCDU- Multi-function Control Display Unit (see below)

*FM- Part of FMGEC FMGEC- Flight Management Guidance and Envelope Computer

個FM compute 出黎ge TOD喺base on 代表d 咩呢?

首先,要明白點先係efficient descent; 意思係由TOD, 收油Thrust Idle由high flight level high speed descent到approach phase with approach speed.

所以, FMS會plot 左飛機喺Vapp (final approach speed) @ 1000ft AGL,respect consider all alt and speed, 再 back track番個Top of Descent position.

如果太早開始descent, 早左到level, 就要喺Lower altitude level flight (嘥油平飛).

如果太遲, 意思係飛機喺high level 飛太耐,遲左descent, 可能要開speed brake 先番到descent profile.

如果headwind/tailwind 會唔會影響個profile?

當然如果unexpected wind, 對managed flight profile 一定會有影響.

入FMS有個norm, Trash in-Trash out.

入錯ge data去個FMS, 佢又點會俾到個accurate computation?

所以我地會盡量入most corrected data去個FMS; 以求most accurate computation.

At last, 有奬問答遊戲! Question to you guys!

如果managed descent, follow 緊個FMS descent profile; 突然ATC instruction 要轉heading, 個descent mode 會點??

comment below~

答得好ge幸運兒,我地會專人contact.

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