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FDs below 200ft AGL


Hugh Morten

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Hugh Morten

Trivial question I suppose but here goes: 

I know that Concorde's flight directors were switched off below 200ft AGL as they were not certified below this height. Presumably this almost certainly did not apply to autolands? How come FDs were fine for autolands and not for manual landings? Surely the technology driving the FD is the same?

Thanks 

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Fraser Gale
7 hours ago, Hugh Morten said:

Trivial question I suppose but here goes: 

I know that Concorde's flight directors were switched off below 200ft AGL as they were not certified below this height. Presumably this almost certainly did not apply to autolands? How come FDs were fine for autolands and not for manual landings? Surely the technology driving the FD is the same?

Thanks 

You are correct that for Autolands the FD's remained in and in fact there was even a runway (roll-out) guidance indicator. 

Autolands in the true sense where visibility is minimal, were only performed on specific certified runways, where the ILS transmitters are kept to a very high level of accuracy.  Some runways were listed as in-between you might say, where autolands were permitted for trial in good visibility, and others were just not certified.  

As you could use the flight director on any approach with an ILS and because there was no way of the system in Concorde knowing how accurate the beams were (unless they weren't good at all and it threw them out) the safest thing was to make a standard procedure where FD's are taken out at 200ft to prevent any issues.  The beams become more sensitive as you get closer to the ground so below 200ft you shouldn't be trying to "hunt" the bars anyway, and should be looking down the runway to check your position and listening to the engineer height calls to monitor rate of descent.

Another factor is that where you could use the FD in VOR LOC and GLIDE mode for a standard, non-coupled approach, LAND mode was used for autolands.  LAND had some slightly different trim rates and tolerances that meant the FD and the autopilot were more "tight" you might say.  I should add here that the autoland didn't use the same landing technique as the humans, hence the mode was also geared to that - also the reason for a slightly longer landing roll, but annoyingly for pilots, much smoother touch down!

I hope that's covered it...!

 

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Fraser Gale
4 hours ago, Hugh Morten said:

Thank you Fraser. If anyone knew the answer, I guessed it would be you!

Not always...

Just as an added extra, LAND mode was actually the default approach mode used because both AP's could be engaged to follow the ILS to decision altitude but it is stated that on lower quality ILS transmitters, VOR LOC and GLIDE should be used.  I would suggest LAND has less damping and follows the beams more literally than the two other modes.  This would also apply to the FD's. 

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Andrew Marshall

My guess is by ILS quality the reference is to ILS CAT I/II/IIIA/B? So appropriate use of FDs and AP modes would depend upon the ILS CAT in use?

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Fraser Gale

In the performance manual each runway was tabulated with the types of approach that were permitted along with whether they were certified for autoland by Concorde. 

In rough terms you are correct in that an ILS only certified for CAT I would be lower quality than those for CAT III but there might have been a "Concorde factor" added on top, I'm not sure.  If I have time I'll try and look it up. 

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Fraser Gale

Just found reference to this in the form that GLIDE mode is intended for use on lower quality ILS beams down to CAT I limits, so LAND mode would be used for CAT II & III ILS beam approaches, as stated in the airport information folders - on an iPad these days!

 

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AdrianSmith

I really can't understand this requirement to switch off the Flight Director below 200' AGL.

As an instrument qualified real world pilot/instructor/examiner my view is that the only thing one should be doing from 200' down on a CAT 1 ILS approach is looking at the runway and manipulating the flying controls (includes throttles).

Anything else is a serious distraction from the critical task of flying/landing the aeroplane in limiting weather conditions.

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Fraser Gale
7 hours ago, AdrianSmith said:

I really can't understand this requirement to switch off the Flight Director below 200' AGL.

As an instrument qualified real world pilot/instructor/examiner my view is that the only thing one should be doing from 200' down on a CAT 1 ILS approach is looking at the runway and manipulating the flying controls (includes throttles).

Anything else is a serious distraction from the critical task of flying/landing the aeroplane in limiting weather conditions.

You are correct and that is what happened...  the non-flying pilot turned off the flight director on a call from the P1 or at 200ft whichever came first.  

The requirement would ensure full attention was transferred to the runway by 200ft although in practice it would have occurred before this in reasonable weather.  

Whether we understand why or not, it's what was done. 

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