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Approach Mode On Autopilot - Questions  
User currently offlineJulesmusician From , joined Dec 1969, posts, RR:
Posted (8 years 11 months 3 weeks 5 days ago) and read 6859 times:

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

15 replies: All unread, jump to last
 
User currently offlineMr.BA From Singapore, joined Sep 2000, 3423 posts, RR: 22
Reply 1, posted (8 years 11 months 3 weeks 4 days 22 hours ago) and read 6841 times:

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!
User currently offlineDavid L From United Kingdom, joined May 1999, 9524 posts, RR: 42
Reply 2, posted (8 years 11 months 3 weeks 4 days 22 hours ago) and read 6834 times:

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


User currently offlineMr.BA From Singapore, joined Sep 2000, 3423 posts, RR: 22
Reply 3, posted (8 years 11 months 3 weeks 4 days 21 hours ago) and read 6829 times:

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!
User currently offlineAAR90 From United States of America, joined Jan 2000, 3475 posts, RR: 46
Reply 4, posted (8 years 11 months 3 weeks 4 days 21 hours ago) and read 6820 times:

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!
User currently offlineDavid L From United Kingdom, joined May 1999, 9524 posts, RR: 42
Reply 5, posted (8 years 11 months 3 weeks 4 days 20 hours ago) and read 6814 times:

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]

User currently offlineDavid L From United Kingdom, joined May 1999, 9524 posts, RR: 42
Reply 6, posted (8 years 11 months 3 weeks 4 days 20 hours ago) and read 6808 times:

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?


User currently onlineCCA From Hong Kong, joined Oct 2002, 838 posts, RR: 14
Reply 7, posted (8 years 11 months 3 weeks 4 days 18 hours ago) and read 6779 times:

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.



C152 G115 TB10 CAP10 SR-22 Be76 PA-34 NDN-1T C500 A330-300 A340-300 -600 B747-200F -200SF -400 -400F -400BCF -400ERF -8F
User currently offlineAAR90 From United States of America, joined Jan 2000, 3475 posts, RR: 46
Reply 8, posted (8 years 11 months 3 weeks 4 days 16 hours ago) and read 6755 times:

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!
User currently offlineDavid L From United Kingdom, joined May 1999, 9524 posts, RR: 42
Reply 9, posted (8 years 11 months 3 weeks 4 days 10 hours ago) and read 6716 times:

More or less as I understood it but, as usual, some interesting extra information. Thanks, guys.

User currently offlineA3204eva From United Kingdom, joined Feb 2004, 1060 posts, RR: 5
Reply 10, posted (8 years 11 months 3 weeks 4 days 6 hours ago) and read 6684 times:

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"
User currently offlineBuckFifty From Canada, joined Oct 2001, 1316 posts, RR: 19
Reply 11, posted (8 years 11 months 3 weeks 4 days 6 hours ago) and read 6678 times:

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.

User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2565 posts, RR: 25
Reply 12, posted (8 years 11 months 3 weeks 4 days 2 hours ago) and read 6632 times:

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.
User currently offlineJulesmusician From , joined Dec 1969, posts, RR:
Reply 13, posted (8 years 11 months 3 weeks 3 days 20 hours ago) and read 6574 times:

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?

User currently offlineAAR90 From United States of America, joined Jan 2000, 3475 posts, RR: 46
Reply 14, posted (8 years 11 months 3 weeks 3 days 20 hours ago) and read 6575 times:

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].



*NO CARRIER* -- A Naval Aviator's worst nightmare!
User currently offlineIAHFLYR From United States of America, joined Jun 2005, 4790 posts, RR: 22
Reply 15, posted (8 years 11 months 2 weeks 5 days 5 hours ago) and read 6416 times:

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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