747-400 Localiser Antenna Switching

[NSEU]

According to my engineering manuals, with APP or LOC selected on the APFD Mode Control Panel, the ILS/MMR Receivers will use the dedicated LOC antennae behind the nose radome rather than the VOR/LOC antennae at the top of the tailfin (courtesy of FCC-controlled coaxial relay switching).

It occurred to me that if the A/P and F/D were off (but serviceable), and you were simply using raw data for your approach, that the received signal would be inferior to that during an AP/FD approach (depending on pitch angle and signal blocking by the fuselage).

Are there any cautions in the training or operations manuals regarding this? Do SOPs force the use of the LOC or APP buttons? (or is the risk minimal)

Thanks
Rgds
NSEU

[Dan Winterland]

The tailfin antenna is the VOR/LOC and will be in use for the whole approach. The antenna behind the radome is the GS receiver. This does switch to GS antanna on th nose gear doors once the gear is lowered. This is according to the Pilot's Manuals. The enginnering manuals seem to say something different. Wouldn't be the first time! There shouldn't be any difference in the signals between manual and automatic flight.

LOC and GS are separate transmissions on different frequencies. LOC is VHF, GS is UHF and the use separate aerials.

[NSEU]

The tailfin antenna is the VOR/LOC and will be in use for the whole approach.

Dan, our books are definitely different. There are even procedures for checking the LOC antenna switching in the Maintenance Manual (34-31-00-735-020) and there are wiring schematics and wiring diagrams which show the relays (in the MEC) and different antenna. The LOC "Track" antennae and the (small) G/S "Capture" antennae are all mounted on the bulkhead behind the radome.

Each FCC controls its respective LOC switching relay.

Even on the Classic there were switchable LOC antennae. I'm sure I've changed one or two LOC switching relays in the distant past (on the MEC equipment rack stanchions). The G/S antennae switching relays are located in the L/H MEC crawlway.

Cheers

[NSEU]

A related question. Not having spent too much time in the jumpseat, I was wondering when the A/P captures the LOC (typically). i.e. how many dots away from the centreline?

I understand it varies depending on groundspeed, intercept angle, etc, but would you ever get capture prior to "localizer alive" (e.g at a large intercept angle, relatively close to the runway). My books tell me you can get Loc capture at up to 120 degrees left or right from the centreline. I see why an omnidirectional tail antenna would be useful in this circumstance (the nose LOC antennae would not pick up the signals at such angles because of shielding).

Thanks
Cheers
NSEU

[main_dog]

As a matter of fact, at typical intercept angles/groundspeeds you usually -but not always- get LOC capture prior to "LOC moving".

Don't quote me, as my memory retains less than your average pasta sieve, but the LOC scale should typically be only 2.5˚ wide each side (so total width 5˚), and that's before it goes into expanded mode.

MD

[NSEU]

Thanks, MD.

I had seen a video on You Tube ("KLM Landing") showing capture at one dot. It involved an overshoot (200kts at 45deg to centerline), but not sure of distance from runway. I've read that shallow intercepts may cause late capture, so I didn't know what to make of this video.

Cheers
NSEU

[crbnftprnt]

NSEU--
The 120 degree figure refers to aircraft heading (possibly ground track; not sure of the logic) rather than aircraft position with respect to the localizer course. That is, localizer capture will occur if intercept angle is within 120 degrees of the localizer course.
crbnftprnt

[grounded27]

I can not confirm this but would say that the size of the aircraft might make GS/LOC reception difficult in a nose up attitude during approach. I am thinking of an MD-11 that has GS ant on the NG and behind the radome. It switches to the gear once gear down.

[lefthanddownabit]

The radome antenna is not just forward of the nose gear door, but higher vertically. The vertical positioning of the GS antenna makes a big difference to the main gear touchdown position relative to the GS transmitter. It also makes a big difference to the visual ground segment (i.e. how far ahead the crew can see at the DH).

The antenna at the nose gear door only becomes usable after the gear is extended. If that wasn't the case there'd be no need to switch them. Since the gear is extended as soon as GS is alive, GS capture and tracking is all done using the optimal antenna location.

[NSEU]

My original questions was more a case of why the Loc Antennae is not switched with a manual (non FD-guided) approach.

I understand the reasons why it is switched ;)

Cheers