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G5 Autothrottle question

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G3G4

Well-known member
Joined
Feb 18, 2006
Posts
139
When the throttles come to idle at 50 feet rad alt, is that a function of rad alt only or the voice call out "50 ft"

Here's what happened, both times at an airport with 5000 ft or less runway.

On approach we got all normal call outs except for the 50 ft call out and both times the throttles did not retard. I'm trying to figure out if we have an auto throttle problem, rad alt problem etc. Any ideas? Or just a glitch in system. Again the both times this happened was flying into a short runway all other times they worked as designed.
 
I don't use the auto-retard feature for two reasons:

1) I still fly the G-IV from time to time and don't want to be in the habit of relying on it (remember the G-IV accident at TEB a few years back? Auto-throttles were ON at touchdown).

2) Every time I watch the other guy use it, the airplane floats and lands long. Every time.

As to your question, as I understand it, the radat triggers the auto-retard and the callout so I'm thinking there's a hiccup in the radar altimeter.
 
Radio Altimeter in the G550:

[FONT=Swiss721SWA,Bold][FONT=Swiss721SWA,Bold]
5. Retard Mode​
[/FONT][/FONT]
The retard mode is used to establish a fixed rate of throttle retardation to the idle thrust
position during the aircraft flare on landing. The retard mode is used for AP/FD coupled
and non-coupled approaches, and it is activated based on radio altitude < 50 ft (i.e., gears
down, flaps > 31.5​
_).
When the aircraft touches down, and both throttle levers are located in the idle thrust stop,
the AT automatically disconnects in preparation for reverse thrust. If the aircraft touches
down without the AT retarding the throttles because of a failure to detect a landing
configuration or lack of radio altitude, the AT retards the throttles to the idle thrust position
when touchdown is detected (i.e.,WOWor both wheels spun up), before it automatically
disconnects. The AT remains engaged until touchdown, so it is available in the event of

a go-around during the flare.
 
Sounds like a problem in your system. I use them (and like them) all the time to touchdown, but on a 5,000 footer may be a little sporty due to float issue that was mentioned.
 
I understand the float issue, I actually wasn't flying. I always like to go to idle at 100 ft on a short runway. But non the less they should have gone to idle at 50 ft rad alt. I was just curious why they didn't when the gpws missed the 50 ft call out. Rad alt was working. Maybe we do have a rad alt problem
 
rad alt issue can be no bueno...

I understand the float issue, I actually wasn't flying. I always like to go to idle at 100 ft on a short runway. But non the less they should have gone to idle at 50 ft rad alt. I was just curious why they didn't when the gpws missed the 50 ft call out. Rad alt was working. Maybe we do have a rad alt problem

There have been some occurences of false 0 indications on the RA causing retard of TLs on approach and ensuing accident, (or maybe just a goat rope). ref: 737 turkish airline accident...
 
There have been some occurences of false 0 indications on the RA causing retard of TLs on approach and ensuing accident, (or maybe just a goat rope). ref: 737 turkish airline accident...

Now I have a third reason why I don't use it. Hadn't thought of it before but I suppose it could anytime you're in full landing configuration.
 
There is a reason why you keep your hands on the throttles and thumb near the AT disconnect when you are using the AT on any approach. As a rule, I disconnect the AT at the 50 ft call just in case and on any gusty approach prior to 500 AGL. Concerning a float with the AT, someone needs to work on their technique. The GV will float massively if you are light and you overflare, so stay ahead of the aircraft, flare at a lower altitude when light and you will do just fine.
 
The system is only as good as its operator. Like the autopilot, never trust the automation when close to the ground, which means that one had is always on the yoke, and the other hand on the throttle (in our SOP) anytime the RA is active. Also, know what mode you’re in, how the automation will comply, and what you’re actually trying to do. Seen people command a descent, while on autopilot, click off the autopilot and level off. All the computers know, is that you want to descend, hence the power is reduced (or at idle), aircraft is level and the pilot is wondering why the throttle aren’t reacting. I always get rid of the flight director when maneuvering visually, say on a visual approach, with no lateral or vertical commands left in the guidance panel, other than speed. Works great, and haven’t been surprised.
 

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