Unknown 737NG Aerials

[CaptainSandL]

I was walking around a 737-700 last week and I noticed these two aerials (right of picture) on the underside of the aircraft. It may be coincidence but they appear to be lined up with the two Rad Alt receive aerials. Does anybody know what they are?

http://www.b737.org.uk/mystery_aerials.jpg

We don't have anything fancy or unusual in the way of avionics on board.

S&L

[Graybeard]

Looks like Boeing took a shortcut to put in the third radalt for triplex autoland. The outboard ones would be for the third radalt.

As holdover from the 737-3/4/5 series, the ones on the centerline are probably lined up T R R T or vice versa. That would make the third set ok for R T alignment (or v-v).

The triplex alignment on a 757 uses the same wedges for the left and right radalts as you show here, while the center radalt is on the centerline. I would post a pic, but it's not on a website, and I don't see how to load it here off my HDD.

GB

[BOAC]

but it's not on a website, and I don't see how to load it here off my HDD.

http://www.pprune.org/forums/showthread.php?t=246758

[CaptainSandL]

Thanks Greybeard, they certainly look like provisions for a third Rad Alt – same size, same distance apart & the wedges give them the same vertical directionality.

The question now is why were they put there? I was not aware of the requirement for a third Rad Alt for triplex autoland but the 737 only has 2 autopilots. The latest versions are Cat IIIb capable with a rudder channel (see other threads) but I am sure a third Rad Alt is not required, it doesn’t appear in the MMEL. Can anyone operating the 737 to Cat IIIb confirm this?

[Graybeard]

Thanks Greybeard, they certainly look like provisions for a third Rad Alt – same size, same distance apart & the wedges give them the same vertical directionality.

The question now is why were they put there? I was not aware of the requirement for a third Rad Alt for triplex autoland but the 737 only has 2 autopilots. The latest versions are Cat IIIb capable with a rudder channel (see other threads) but I am sure a third Rad Alt is not required, it doesn’t appear in the MMEL. Can anyone operating the 737 to Cat IIIb confirm this?
Are you sure your plane doesn't have a third radalt?

The radalt has become a more vital part of safety fixes over the years, such as making sure the plane is on ground before the reversers deploy: Lauda 767. The Boeing triplex philosophy for critical systems is really a 2 of 3 voting system.

3 ILS receivers, 3 radalts, and 3 IRU feed 3 Fright Control Computers, who do a continuous voting during approach. In the 757/767, the voting is carried all the way to the control surfaces, where the hyd power units play in unison in detent. If one computation/command is out of unison with the others, it pops out of detent, and the approach is continued with dual command.

Airbus, OTOH, has the dual-dual architecture, as did the first commercially successful Cat IIIb, the L-1011, and the inept DC-10 and less inept MD-11. It takes much higher integrity sensors, ILS, IRU and radalt, to assure safety in a dual-dual configuration.

I'm not really familiar with the 737-7 series, but they possibly use triplex sensors to feed the dual fright control computers. It would be a step toward full triplex.

GB

[CaptainSandL]

No definitely no third Rad Alt - and it has never had one, we have had the aircraft from new. No signs of a gap or alteration in the in the c/b’s either.

The 737 A/P’s are dual-dual and we only have 2 FCC’s, IRS’s, ILS receivers and Rad Alts. I don’t see where a third would fit into the system, unless it was a backup for Rad Alt #1.

Have any other 737NG operators noticed if they have these blank extra Rad Alt aerials? They could have been standard fit in anticipation of a development or other reason that has since become redundant.

[Graybeard]

The 737 A/P’s are dual-dual and we only have 2 FCC’s, IRS’s, ILS receivers and Rad Alts. I don’t see where a third would fit into the system, unless it was a backup for Rad Alt #1.

Have any other 737NG operators noticed if they have these blank extra Rad Alt aerials? They could have been standard fit in anticipation of a development or other reason that has since become redundant.

The manufactures have sometimes leased back a new airplane for flight testing, and they could have added the third radalt. If so, they must have decided to leave the antenna mounts in place, rather than remove them and plug the holes.

Slight clarification: the Cat IIIa autoland, such as the MD-80 and 737 are simply dual, not dual-dual. They don't have the full redundant circuitry within each Fright Guidance Computer, as they are fail passive. If there is a failure or mis-compare, they give the plane back to the pilot. A Cat IIIb autoland will continue the approach and landing after a single failure, which requires dual-dual circuitry. Note, that does not mean 4 sensors each, but just two. The Triplex autoland uses 3 relatively simple Flight Control Computers and 3 sensors each.

There was no point making the 737 dual-dual or triplex, until it had a fail operative rudder.

GB

[Drop The Dunlops]

The manufactures have sometimes leased back a new airplane for flight testing, and they could have added the third radalt. If so, they must have decided to leave the antenna mounts in place, rather than remove them and plug the holes.

Slight clarification: the Cat IIIa autoland, such as the MD-80 and 737 are simply dual, not dual-dual. They don't have the full redundant circuitry within each Fright Guidance Computer, as they are fail passive. If there is a failure or mis-compare, they give the plane back to the pilot. A Cat IIIb autoland will continue the approach and landing after a single failure, which requires dual-dual circuitry. Note, that does not mean 4 sensors each, but just two. The Triplex autoland uses 3 relatively simple Flight Control Computers and 3 sensors each.

There was no point making the 737 dual-dual or triplex, until it had a fail operative rudder.

GB


But the newest 737's ARE Dual-Dual, and are certified to Cat IIIb standards. AFAIK most 737NG aircraft post c.2004 are kitted out for CAT IIIb, to enable them to compete more effectively with the Dual-Dual A320s.

[CaptainSandL]

Since Feb 2003 737s have been produced with the Collins EDFCS-730, this is fail operational, has an optional rudder channel and is Cat IIIb certified. I presume that these aircraft are dual-dual, even if the previous aircraft were not.

We operate a mix of Collins (Cat IIIb) and the earlier Honeywell (Cat IIIa) 737NGs. The photo was taken of a Honeywell equipped aircraft and I have seen these on a couple of other Honeywell aircraft since. I am now wondering if these fittings (pretty sure now that I can’t call them aerials) are on our Collins equipped aircraft.

[alexban]

I've seen that for fail operational NG's ,the annunciator can indicate LAND 2 or LAND 3. What does this mean? maybe 3 a/p?
We don't have fail op Ng's ,this is only from the FCTM.
Brgds

[airmen]

Why dont you call Mr. Boeing with your aircraft S/N and ask him directly...?
You'll have the correct answer then!

[CaptainSandL]

Hi Alex,
As I understand it “NO LAND 3” on C/R means Cat IIIb is not available, “LAND 2” means Cat IIIa is still available. “NO AUTOLAND” is the other caption that may be seen. Definitely only 2 A/P’s on the 737, we know this because the Collins MCP only has 2 A/P switches.

Airmen,
That would be my last step. If I went to Boeing with every question I had about the aircraft they would soon tire of it. However if it gets to that stage I shall post the answer here. We have a lot of 737 guys on PPRuNe, I just thought someone out there might know and it would be of interest to the others.

S&L

[JackOffallTrades]

You must be a short guy to go walking under there!