Boeing (744/8) Localizer "sensing"

[RandomPerson8008]

Hello,

B744/748.

Is there any prohibition or harm in arming the LOC prior to having "correct sensing" i.e. when the localizer pointer is still sensing in reverse due to the aircraft not yet tracking within 120 degrees of the localizer course? Example being a Localizer approach with a procedure turn: should/may you arm localizer prior to the inbound portion of the procedure turn?

As I understand it, the aircraft will not inadvertently capture the backcourse. The systems manual states "localizer capture can occur when intercept track angle is within 120 degrees of the localizer course".

Additionally, the FCTM states:

APP mode should not be selected until:
-The ILS is tuned and identified
-The aircraft is on an inbound intercept heading
-Both localizer and glideslope pointers appear on the attitude display in the proper position
-Clearance for the approach has been recieved.

Notice it says "APP" mode, not "LOC" mode, although logically the implications would be the same for the LOC mode.

My apologies if I missed this answer elsewhere in the manuals.

Personally I don't arm the LOC or approach modes until I have correct sensing as a matter of airmanship but I hesitate to tell others to do the same without a reference. I am attempting to build a case to dissuade people from premature LOC selection during a non-ILS, but maybe I'm wrong about this.

Thanks.

[Roj approved]

The Boeing instructors on the 787 back in 2013 were big on:

C - Cleared

H - Heading to intercept

I - ILS on PFD

P - Pointers in correct sense

But I can't remember if it "HAD TO" be in the correct sense or whether it was just good airmanship.

[Fursty Ferret]

I really wouldn't do it while the localiser is showing the wrong direction, if for no other reason that it might then not turn onto the inbound course when the pointer flips to the correct side. Or turn the long way around to the inbound course.

[CCA]

You can arm and capture with incorrect sensing it will turn correctly.

However, I too wait for correct sensing though as usually it means I’m closer to the runway and less likely to suffer the following.

Flight crews should exercise caution to possible false capture of localizer and flight check procedures should be designed to reduce the risk of this type of event by not allowing the Flight Director/Autopilot capture modes to be armed too early.

[speed freek]

Notice it says "APP" mode, not "LOC" mode, although logically the implications would be the same for the LOC mode.

I would argue the reason there’s a difference is if LOC captures the wrong localiser, you can engage another roll mode and recover.

If using APP and it locks onto the wrong ILS, you’d have to select TOGA or turn off the A/P and F/Ds and then rebuild, all while turning the wrong way, possibly towards hills. Bit messy.