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737 Navigation Question On ADF/ HSI

Thu Nov 13, 2008 6:27 pm

Hi there!

I'm just doiung my (Maintenance Personel) Boeing 737CL Typerating.

Yesterday I had to write an exam about ATA 22 and ATA 34. I had a great success by having 31 right answers out of 33. One of the questions I answered wrong was about the ADF needle that can be shown on the EHSI (Electronic Horizontal Situation Indicator = lower display).

The question was: "What happens to the ADF needle on the EHSI when there is no reception on the selected ADF frequency"

a) it is not displayed
b) it is shown parked at the 09:00 o'clock position
c) it has no special position

Answer "b" was pointed out to be correct by two instructors. I've choosen "a" though...

I'm very sure that the needle is not displayed anymore.
It's clear that on the RDDMI the needle would be in the parked position ( 09:00 o'clock) since it cannot be removed on an analogue instrument.
But from my experience on the 737 of Lufthansa and my 737PIC I use to fly on my flightsim it is not shown if no frequency (within the range of reception) is selected.
[Lufthansa has no ADF selectable on the RDDMI (only 2 VOR needles). But on my sim the 737s have the RDDMI where each needle can be used vor VOR or ADF display.] And in each mode (FULL VOR/ILS; EXP VOR/ILS; etc) the ADF needle disappears if an ADF FREQ wihout reception is selected.
According to the instructor only on the 737NG and A320Fam. (talking about short range aircrafts here...) ADF needles are removed from the displayes when there is no reception.

Hence the course will be running for more than a week from now I don't have the time to jump into on of our Bobbies. So your experience is highly welcome!

WILCO 737 has a great image up here for this discussion. HSI is seen here and the RDDMI is pointing out 2 VOR/DME stations (no ADF selectors on LH 737s!).

Txs in advance!


[Edited 2008-11-13 10:32:53]
Life starts at take-off!
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RE: 737 Navigation Question On ADF/ HSI

Fri Nov 14, 2008 11:05 am

Hi Stephan,

I had a quick look into my documents (OM-B, Chap. 12.11.30) to check (didn't want to rely on my memory) and you're right. On page 1 is stated that in case of ADF receiver failure the pointers are removed and on page 8, what is your question, that the pointer is only displayed if an ADF bearing is being received.
That's also what I remembered from flying the Bobby.

Good luck for the rest of your typerating,

Posts: 412
Joined: Wed May 15, 2002 12:26 am

RE: 737 Navigation Question On ADF/ HSI

Fri Nov 14, 2008 2:58 pm

Thank's a lot Chris!

Since we have the OM-B in our school too I'll show it to the instructor! Great to know that this is pointed out in the on-board-docs as well.

Life starts at take-off!
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RE: 737 Navigation Question On ADF/ HSI

Sat Nov 15, 2008 6:16 pm

I would choose "a" as well, based on my knowledge of other EFIS aircraft. I'd be curious to know if "ADF failure" and "no reception" are the same thing, and handled the same by the system. I know of a case where DME failure, and DME weak/no signal are displayed differerntly.

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