Third radio altimeter on B737NG
 

Hi all,

Is there anybody to let me know optional third RA's purpose and function?

Also, if third radio altimeter system is installed, where is the radio altitude shown?

I've not seen or heard of a third Rad Alt in B737NG...

Hi friend,

Most B737NG do not have the third RA system as you said. However, I hearded that It can be optionally installed. Two B737NG airplanes my company operate have two raised pads on the bottom fuselage near radio altimeter antennas. Please refer to blow.








Radio Altimeter On 737 NG — Tech Ops Forum | !!!!!!!!!!!!!!

Wouldn't surprise me, the options list for a 737 is quite long and has quite a few rarely if ever ordered options. However, the third RA is not needed for fail operational installations as is suspected in the thread you linked. Two RAs are sufficient for that.

Hi, third RA in other Boeing airplanes (i.e.B767) is used by the Center AP. There is no display for it, unless the CP or F/O switches his Simbol Generator to Altn.

Hi all,

Thanks for your responses. I want to know the third RA of B737NG in detail. Please let me know.

I would think 3 R/As would be required for CatIIIB?

Why would it? The 737 only has two autopilots, even for fail operational (or cat iiib) operation.

Hi all,

The reason why I would like to know the third RA is because of MMEL34-20. Refer to below.

SYSTEM / INTERVAL/ NO. INSTALLED / NO. REQ'D FOR DISPATCH

2) Indications, C, ㅡ, 2

May be inoperative provided
a) Independent radio altimeters operate normally for both flight crew
members.
b) Approach minimum do not require its use

C, ㅡ, 0

May be inoperative provided
a) Associated receiver/transmitter is verified to operate normally, and
b) Approach minimums or operating procedures do not require its use.


As you know, "ㅡ" symbol indicates a variable quantity of the item installed. So I am very confused because I have seen only two RA indications displayed on Capt side and F/o side. However, MMEL say that the number of RA indications can vary.

Therefore, I guess that it may be related to the third RA.

How do you think?

No such thing as a third RA on 737NG. You only need one for dispatch and 2 to do Cat 11 or Cat 111 approach.