Intercept LOC in HDG

[Twin2040]

Hi there. In my previous company SOP it dictated always to intercept LOC in HDG mode, even if going to a FMS point located straight on long final. In the the company I fly now, no such thing in the SOP - however if I look in the FCOM its still HDG when intercepting LOC - what are the reason for intercepting in HDG mode and not NAV from the FMS - Thanks

[MD80rookie]

Maybe to avoid flying parallel to the LOC in NAV mode without capturing it? In HDG you "force" the aircraft to capture it.

Regards,
Rookie

[Twin2040]

Could be .... but flying straigt towards a FMS point located on final with a 40 degrees intercept - "old" SOP still wants you to abandon NAV and select HDG whan cleared approch and then arm LOC/ILS mode.

[MD80rookie]

Yep, and you increase your situational awareness by actively monitoring that the aircraft is turning BECAUSE your capturing the loc and not because you're following the inbound course towards the rwy by the FMS.

[Twin2040]

Thanks - must be it then - also on MD80 by the way ....

[Microburst2002]

It could have to do with the lead in to the turn.

LOC capture gains are such that the capture will be quite aggressive, as if outside the beam it was full of "Mordor like" mountains and obstacles.

In HDG the airplane will wait longer until it starts turning than in NAV or LNAV. If you are fast it will probably overshoot once and then recapture also aggressively and then maybe overshoot again once before capturing accurately.

In NAV or LNAV mode the turn initiation will be speed sensitive, so if you are really fast it will start turning very early, maybe even with small bank angles and capture without overshoot, and somehow that is not desirable because it would be dangerous in that world of killer obstacles and terrain outside of the ILS beam. That's what I think.

In VMC or well above MSA I favor a NAV, gentle capture when I fly fast, rather than a HDG capture.

[aterpster]

Microburst 2002:

In NAV or LNAV mode the turn initiation will be speed sensitive, so if you are really fast it will start turning very early, maybe even with small bank angles and capture without overshoot, and somehow that is not desirable because it would be dangerous in that world of killer obstacles and terrain outside of the ILS beam. That's what I think.

The protected airspace is considerably wider than the ILS "beam."

[Microburst2002]

I know, but I think that the reason of the AP "aggressive" way of capturing the LOC is keeping the airplane well inside the protected area.

The AP will capture any LOC in the same manner, no matter if it is in the flattest airport ever or in the most terrain restricted airport.

This is what I think it happens, as I said. I expect some expert would enlighten us about that, as it is usual in this forum

[Natstrackalpha]

Strangely enough, I find that unless I very carefully provide the intercept angle and the speed to boot and as early as I can get away with, subject to ATC and traffic and MSA and OCL the aircraft will engage in LOC and fly straight through the LOC centreline and try to re-intercept the same only to be doing a rather fancy series of turns until too late on final when a manual landing or a go-around is a must, usually the latter (mainly because my glide has gone up the Swannee by this time too).

In fact, I find, if I don`t get it set up at precise distances/speeds and config then I know I am in for a rather boring time. Unless I am at 20 plus miles, I would not trust / try intercepting when `fast` especially with anything heavy like an A330 and not for an 320, maybe a 319, but now am getting philosophical, and even then I could end up unpleasently surprised. It might work for a C150 though.

[baobab72]

hi

my two cents

it depends on the aircraft that you fly, if you have a nav to nav xfer feature as we do in the Collins proline 21 a nav to nav transfer is possible, however even though that is not contemplated in our SOP's, i had several instances when despite of the fact i was at the recommended speed - 180kts - and with a fair interception angle - 30 to 45° -, the interception was very sloppy so i switched to HDG mode. This seems to occur more often at certain airports - UKBB Kiev comes to mind -, especially when intercepting from afar and with a reduced interception angle. I do also revert to HDG when flying in high altitude terrain environment - such as LOWI Innsbruk, cos i want to make sure that the AFCS will intercept the localizer before hitting the mountain!
Finaly for passenger comfort sometimes during the nav to nav transfer as the nav source is changed from magenta - fms - to green the airplane banks slightly as the sensitivity is changed.

In the 605 you can not display the DME distance unless the nav source is set to a LOC or VOR so by switching to green source and then putting a bearing on the IF - two waypoints prior to the threshold - allows you to have a better situational awareness.

Happy landings

Baobab72

[PPRuNeUser0190]

In my previous company it had to do with IRS drift. On the B737 CL we always intercepted in HDG SEL to avoid parallelling of the localiser in LNAV due to position shift. That combined with some CL being able to follow a glideslope without being localiser captured... You see the point :)

On NG, accuracy is that precise that large position shift is very unlikely.