Boeing 737 Autothrottle System
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Boeing recommends AT use only when the autopilot is engaged.
The autothrottle ARM mode is normally not recommended because its function can be confusing. The primary feature the ARM mode provides is minimum speed protection in the event the airplane slows to minimum manuevering speed. Other features normally associated with the autothrottle, such as gust protection, are NOT provided. The autothrottle ARM mode should not be used with Non-Normal checklists. Some malfunctions that affect manuevering speeds cause the autothrottle to maintain a speed above approach speed.
When using the autothrottle during autoland, position command speed to VREF +5 knots. Sufficient wind and gust protection is available with the autothrottle engaged because the AT is designed to adjust thrust rapidly when the airspeed drops below command speed while reducing slowly when the airspeed exceeds command speed. In turbulence, the result is that average thrust is higher than necessary to maintain command speed. This results in an average speed exceeding command speed.
During sporty landing conditions, my experience is that the most skilled aviators, without exception, prefer to disconnect the Boeing 737 "hunt-o-matic" autothrottle system.
No wonder it is the only autoflight system Boeing will allow to be inoperative for a Cat III autoland.
Bottom line: You squat to pee, you autothrottle to land
The autothrottle ARM mode is normally not recommended because its function can be confusing. The primary feature the ARM mode provides is minimum speed protection in the event the airplane slows to minimum manuevering speed. Other features normally associated with the autothrottle, such as gust protection, are NOT provided. The autothrottle ARM mode should not be used with Non-Normal checklists. Some malfunctions that affect manuevering speeds cause the autothrottle to maintain a speed above approach speed.
When using the autothrottle during autoland, position command speed to VREF +5 knots. Sufficient wind and gust protection is available with the autothrottle engaged because the AT is designed to adjust thrust rapidly when the airspeed drops below command speed while reducing slowly when the airspeed exceeds command speed. In turbulence, the result is that average thrust is higher than necessary to maintain command speed. This results in an average speed exceeding command speed.
During sporty landing conditions, my experience is that the most skilled aviators, without exception, prefer to disconnect the Boeing 737 "hunt-o-matic" autothrottle system.
No wonder it is the only autoflight system Boeing will allow to be inoperative for a Cat III autoland.
Bottom line: You squat to pee, you autothrottle to land
Per Ardua ad Astraeus
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I'm sure this has been covered on PPrune before, but if you work out Vref+5 and 1.3 Vstall at various weights (737) at Flap 40 in particular, there are certain combinations where the 'ARM' mode will keep on 'cutting in' if you fly ACCURATE speeds. It is a pain and I am (in a non-MCP way 
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Amazing isn't it. Boeing spend thousands of dollars on testing a system and then publish a recommendation based upon the research into all the pros and cons and then operators think they know better. Why is it that Boeing advice is thrown aside ?
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Hudson, you need to remember that
- Manufacturers don't operate the aircraft day in, day out
- Operators will probably have several types (and several variants) on their fleets and need to have fleet commonality of SOP's and operating methods
- Operators are subject not only to the manufacturers' recommendations but also their local (national) regulatory authorities
- etc.
