747-400 glideslope antenna

[Jon Lei]

Can someone pls tell me the different locations of the B744 glideslope antennas and when will the automatic switch over takes place during the approach. Cant seem to find it in the fcoms.

Thanks in advance.

[keel beam]

The ILS capture antennas are under the radome, centre antenna is placed above the weather radar antenna and the left/right dual antenna is below the weather radar antenna. The change over occurs when the landing gear is lowered. The left/right dual antenna is on the aft left nose wheel door and the centre on the right aft nose door.

[gengis]

Jon, there are 2 types of glideslope antennae on the -400. 1st one, the "Glideslope Capture Antenna" is located in the radome. This is used during the intercept of the glideslope & during the capture. After the gear is extended, the system switches to the "Glideslope Track Antenna" located on the forward part of the nose gear door, which gives a slightly better correlation of the position of the nose gear to the localiser than the capture antenna (due to the size of the airplane). This is the one used for the remainder of the approach. The reason that the "capture" antenna is used for the initial part of the approach is that prior to nose gear extension the track antenna is of no use.

It is an interesting point, because in a max crosswind approach, if you use the crab technique, the amount of drift that you lay off to track the centreline will actually give a slightly upwind of localiser indication, since the nose gear will be significantly displaced due to the crab angle. I have noticed this. However, if you use the sideslip technique, or watch the performance of the autopilots in an automatic-coupled approach where the system adopts a partial sideslip during the later part of the approach, the localiser indicates on centreline due to the reduced crab angle in a sideslip approach, leaving the nose gear much closer to the centreline.

[Marty-Party]

Just for interest, where are the localiser antenna on the 747-400?

[gas path]

VOR/LOC antenna on tip of the fin and 2 dual LOC antennas on the fwd p/x bulkhead (one above and one below the w/x radar scanner).

edit to add: The fin antenna is for in the air prior to the approach and the 2 dual antennas for the approach mode.