julesmusician
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Approach Mode On Autopilot - Questions

Thu Nov 03, 2005 6:56 am

I want to know what real aircraft do as Flight Sim seems to make up its own mind on what it wants to do and does different things...here are the questions

1) At what point can the Approach mode be set to "ON" anytime up to arriving at the point of catching the ILS or on the point of catching the ILS?

2) Should the autopilot switch the HOLD mode on the ALTITUDE "OFF" automatically when it catches the ILS thereby allowing the autopilot to determine both altitude and heading? Or should the pilot turn it off when he captures the ILS?

For some reason Flight Sim seems to turn the altitude hold off when flying a B737 but leaves it on on the B747....

what happens in real life?

Cheers,

J
African Civil Aviation Commission president "You don't want to fly out as a passenger and come back as cargo."
 
Mr.BA
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RE: Approach Mode On Autopilot - Questions

Thu Nov 03, 2005 8:26 am

For a start in the default flightsim the autopilot is totally different from reality so I think it'll be difficult to compare.

Cheers.
Boeing747 万岁!
 
David L
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RE: Approach Mode On Autopilot - Questions

Thu Nov 03, 2005 8:53 am

Not meant as an answer (not qualified for that) but I'd like to link a version of the same question from my point of view if I may.

Basically, is it correct to say that the LOC mode will follow the selected heading and altitude until it captures the localiser, at which point it will turn the aircraft on to the localiser while maintaining the selected altitude? Selecting the Approach mode will hold the selected altitude until the glideslope is captured, at which point it will cause the aircraft to descend on the glideslope? And it's normal to be centred on the localiser before arriving on the glideslope?

Quoting Mr.BA (Reply 1):
For a start in the default flightsim the autopilot is totally different from reality so I think it'll be difficult to compare.

Think "PMDG".  Smile
 
Mr.BA
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RE: Approach Mode On Autopilot - Questions

Thu Nov 03, 2005 9:21 am

By engaging LOC, the aircraft would capture the localiser while maintaining the selected altitude or continue climbing or descending. When APR is engaged, the aircraft would capture the glideslope, irregardless whether you're climbing or descending. Usually this switch is engaged when the glideslope is "alive" and the autopilot would 'catch' the glideslope by maintaining the diamond in the centre. If the aircraft is too high, the diamond would appear lower than the centre and vice versa. The autopilot will always try to maintain it in the centre. If you're further away from the runway at an attitude say 2000, the GS will only come 'alive" approximately about 3 nm from the runway. But say you're at 6000, it'll come "alive much further out about 8 nm.

There are dangers capturing the glideslope without the localiser (deviated from the approach path). There could be high terrain.. etc.

[Edited 2005-11-03 01:22:31]
Boeing747 万岁!
 
AAR90
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RE: Approach Mode On Autopilot - Questions

Thu Nov 03, 2005 9:34 am

The answer to your questions are: it depends upon the equipment installed on the airliner being flown.

Quoting Julesmusician (Thread starter):
1) At what point can the Approach mode be set to "ON" anytime up to arriving at the point of catching the ILS or on the point of catching the ILS?

Most airliners I've flown, you can "arm" the APProach mode at any time. Some will accept arming with no localizer freq. tuned, others will not.

Quote:
2) Should the autopilot switch the HOLD mode on the ALTITUDE "OFF" automatically when it catches the ILS thereby allowing the autopilot to determine both altitude and heading?

Most autopilots do not enter a HOLD mode upon glideslope capture, but instead follow the glideslope guidance with no reference to altitude.

Quote:
Or should the pilot turn it off when he captures the ILS?

I know of no airliner autopilot that requires a pilot to do anything at localizer and glideslope capture(s). That's why they are "armed" prior to capture... permitting pilot(s) to monitor proper autopilot following of course & glideslope.
*NO CARRIER* -- A Naval Aviator's worst nightmare!
 
David L
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RE: Approach Mode On Autopilot - Questions

Thu Nov 03, 2005 10:01 am

Quoting Mr.BA (Reply 3):
while maintaining the selected altitude or continue climbing or descending

Fair point - I really just meant that the Localiser mode doesn't have any say about the altitude.

But, generally, you use the Localiser mode first to align yourself laterally with the runway and then Approach mode to continue on the localiser and then descend down the glideslope?

I guess Localiser mode shouldn't be engaged until you're on a reasonable intercept course and the system can see the localiser (or will soon?) and Approach mode shouldn't be engaged until you're also happy that the glideslope is visible to the system (or will be soon?) and that you're progressing in such a way that you can get yourself centred on both without much difficulty.

Julesmusician: I hope my hijacking of your question hasn't stopped you getting the answers you wanted. I'm going to shut up now, just in case.  

Edit: Sorry, AAR90, I took so long to compose this reply that you'd already answered some of my questions!

[Edited 2005-11-03 02:04:19]
 
David L
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RE: Approach Mode On Autopilot - Questions

Thu Nov 03, 2005 10:09 am

Quoting AAR90 (Reply 4):
Most airliners I've flown, you can "arm" the APProach mode at any time. Some will accept arming with no localizer freq. tuned, others will not.

So, if Approach mode is armed but no ILS signal is detected, the autopilot will continue to use the current Heading (or LNAV?) and whatever vertical mode was in effect before APP was armed?
 
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CCA
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RE: Approach Mode On Autopilot - Questions

Thu Nov 03, 2005 12:33 pm

Quoting David L (Reply 6):
So, if Approach mode is armed but no ILS signal is detected, the autopilot will continue to use the current Heading (or LNAV?) and whatever vertical mode was in effect before APP was armed?

Correct.

However there is also the possibility of a false LOC capture this is why it pays to careful when arming APP. Some airports have NOTAMS saying you can expect false LOC captures outside a certain distance like 25nm or outside 5 degrees of the LOC due to such things as terrain.

Also the ILS could be off or on test but the AC doesn't know this, it only knows Localizer frequencys which are between 108.1 and 111.95 mHz.

Just to add to what AAR90 wrote the 747 classic AP will drop from CMD to MAN if you select ILS on the NAV mode switch on the mode control panel without a Localizer frequency, in MAN on the classic, depending on the AC you will end up in CWS, (control wheel steering) or in AP manual control using the manual control panel.
P1 in A330, A340, A346, B742, B744, B748.
 
AAR90
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RE: Approach Mode On Autopilot - Questions

Thu Nov 03, 2005 2:09 pm

Quoting David L (Reply 6):
So, if Approach mode is armed but no ILS signal is detected, the autopilot will continue to use the current Heading (or LNAV?) and whatever vertical mode was in effect before APP was armed?

Don't know other acft (thankfully old plane information is slowly seeping into memory cells I can no longer find  Wink ), but AA's 738 A/P will continue operating whatever mode it was in when APP mode is armed and no valid signal AND guidance information is detected (for whatever reason). You can change the operating mode {LNAV to HDG SEL for example} without disarming APP mode. OTOH, you can "disarm" APP mode {or any other "armed" mode} by pressing its button again prior to that mode going "active." A nice design feature that attempts to keep inattentive pilots out of trouble {i.e. don't change anything until the "armed" mode goes active}.  Wink
*NO CARRIER* -- A Naval Aviator's worst nightmare!
 
David L
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RE: Approach Mode On Autopilot - Questions

Thu Nov 03, 2005 8:05 pm

More or less as I understood it but, as usual, some interesting extra information. Thanks, guys.
 
A3204eva
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RE: Approach Mode On Autopilot - Questions

Fri Nov 04, 2005 12:27 am

And set a heading 30º from the rwy course for intercept. And always establish at a distance where you can fully line up with the rwy then travel down the loc for a mile or so before hitting the GS  Smile
"They have lady pilots......... they're not that good, but they have 'em"
 
buckfifty
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RE: Approach Mode On Autopilot - Questions

Fri Nov 04, 2005 12:43 am

Another simple explanation; we only arm the approach mode only when we're cleared for the approach specified. And as previous explanations have alluded to, we make sure the approach aids are all correctly tuned and identified before we arm the APPR mode.
 
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Jetlagged
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RE: Approach Mode On Autopilot - Questions

Fri Nov 04, 2005 4:00 am

Quoting Julesmusician (Thread starter):
2) Should the autopilot switch the HOLD mode on the ALTITUDE "OFF" automatically when it catches the ILS thereby allowing the autopilot to determine both altitude and heading? Or should the pilot turn it off when he captures the ILS?

Yes, when G/S is captured the glideslope guidance will take over pitch control, so ALT HOLD or ALT SEL certainly will trip OFF. Exactly the same as the way HDG SEL or HDG HOLD trips off when LOC captures.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
julesmusician
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RE: Approach Mode On Autopilot - Questions

Fri Nov 04, 2005 10:22 am

Thanks all very useful answers - Flight Sim still has the ID mode and you can listen to the morse code of the nav beacon you are on - is this still used ever?
African Civil Aviation Commission president "You don't want to fly out as a passenger and come back as cargo."
 
AAR90
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RE: Approach Mode On Autopilot - Questions

Fri Nov 04, 2005 10:34 am

Quoting Julesmusician (Reply 13):
Flight Sim still has the ID mode and you can listen to the morse code of the nav beacon you are on - is this still used ever?

Tune AND identify is still required; however, AA's 738 FMS' have a function that will display the navaid identifier so a pilot does not have to actually listen to the morse code ID IF the proper ID is displayed. A nice feature I'm sure is on most newer model FMS'. If no ID is displayed, or the wrong ID is displayed, the pilot(s) must listen to the proper morse code ID prior to using the navaid for navigation [happens once or twice a year].
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IAHFLYR
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RE: Approach Mode On Autopilot - Questions

Thu Nov 10, 2005 1:01 am

Jules think PMDG cost isn't much and a great add-on!
Any views shared are strictly my own and do not a represent those of any former employer.

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