Asymmetric Committal Height

What is Asymmetric Committal Height(ACH)?

The minimum height from which an asymmetric approach may be abandoned.

What is the significance of ACH?

In the event of an asymmetric go-around, the aircraft will take some time to clean up (ie, approach flaps up, landing gear up) before climbing away. The transition time for approach configuration to best single engine rate-of-climb (Vyse) will involve some height loss due to poorer aircraft performance (ie climbing on one engine) and relatively high drag from the dead engine. Therefore, with ACH, there will be sufficient time for the aircraft to maintain or accelerate to Vyse and still be able to maintain directional control whilst cleaning up.

What factors affect ACH?

Height varies with the following factors:-

Aircraft Type- How much thrust can the engine provide.
All Up Weight- More power required for higher AUW.
Altitude of the aerodrome- Aircraft performance will decrease when the aerodrome is located at a higher elevation
Air Temperature- The warmer the air, the less dense it is, therefore, aircraft performance will decrease
Height of obstruction along the take off path- With taller trees or power lines, it will be more challenging therefore, higher ACH should be nominated.
Pilot Skills- Less experienced pilots should nominate a higher ACH.

What height should ACH be?

When conducting asymmetric training or testing, a rough gauge of 300ft AGL should be used. However, it can vary with the different factors. In the event of a real asymmetric condition, ACH may be lowered to ensure a landing is possible. In such an event, the approach speed should be maintained above Blue Line speed (ie, Vyse) until a visual landing is assured.

What happens after ACH?

The full flap setting should not be selected until the pilot’s final decision is to land which should happen at ACH. Once below ACH, a pilot should be committed to land. He/She could use the following guide in making the decision:
- On speed, on profile, on centerline
- Configured to land (ie, landing flaps down, landing gear down)
- Runway is clear
- Clearance from ATC to land has been received.

References
1. Global Air Services (2009). Training Manual Part 2. Retrieved from http://www.ypa.gr/up/files/D2_Isxyonta_Enypa_Ptyxia/Global_Training_Manual_Part2.pdf on 10 August 2011
2. Wycombe Air Centre (2010). Flying Order Book. Retrieved from http://www.wycombeaircentre.co.uk/members/FOB-2010.pdf on 10 August 2011
2. Civil Aviation Authority (2009). HandlingSense Leaflet Twin Piston Aeroplanes. Retrieved from www.skybrary.aero/bookshelf/books/655.pdf on 10 August 2011
2. Civil Aviation Authority Czech Republic. Interpretative and Explanatory Material. Retrieved from http://www.caa.cz/file/2306_1_1/download/ on 10 August 2011

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Elisha Lim


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