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Temporary loss of VOR/LOC signal on approach

I have noticed that I occasionally have a “glitch” in the signal on a VOR or ILS approach, resulting in a NAV flag for maybe a second or two. It might happen once or twice on an approach, or not at all. What are the rules about continuity of signal for a conventional approach? Specifically, when would such a failure require going missed?

EHRD, Netherlands

@dutch_flyer am sure you have thought of this, but were you inside the DOC of the ILS?

Oxford (EGTK), United Kingdom

RobertL18C wrote:

@dutch_flyer am sure you have thought of this, but were you inside the DOC of the ILS?

I’m quite sure I’ll get an earful for this question (“how can you possibly fly IFR safely without knowing what DOC is??!!”), but in 20 years I’ve never heard the acronym “DOC” related to an ILS. And it sure is hard to Google given that “doc” has a quite common alternative usage. I can only assume it means “[something] of coverage” or something along these lines.

Please enlighten me…

EHRD, Netherlands

Designated Operational Coverage quite a standard question at least in FAA.

Oxford (EGTK), United Kingdom

Designated Operational Coverage

Localizer Coverage and Integrity: The coverage and validity of ILS localizer
signals are regularly confirmed by flight inspection within 35° of either side of a
front- or back-course nominal approach path to a distance of 10 NM, and through
10° of either side of a front- or back-course nominal approach path to a distance
of 18 NM
/// The glide path transmitter operates within the frequency range of 329.3 to
335.0 MHz. The frequency is paired with the associated localizer frequency in
accordance with ICAO standards. The glide path is normally adjusted to an
approach angle of 3° and a beam width of 1.4°. There is no usable back course.
The antenna array is located approximately 1 000 ft from the approach end of the
runway and offset approximately 400 ft from the runway centreline.
At some of the larger airports, an ILS is installed at each end of a runway. In this
way, a front course approach may be made to either end of the runway. The two
systems are interlocked so that only one ILS can operate at any time.


always learning
LO__, Austria

Ok so I’m obviously familiar with the concept, but I didn’t know the acronym. My question was about being on a localizer or VOR on approach, meaning on final with a centered needle, then experiencing a temporary loss of signal. So I don’t think DOC is relevant in this case.

EHRD, Netherlands

Possibly a loose antenna connection? Shouldn’t happen and the ground stations are monitored by ATC, so probably worth checking. The fact it happens on both VOR and LOC might suggest antenna?

Oxford (EGTK), United Kingdom

dutch_flyer wrote:

What are the rules about continuity of signal for a conventional approach? Specifically, when would such a failure require going missed?

I don’t know if there are any rules. It would depend on the pilot’s judgement. Personally, I wouldn’t care. Nothing much can happen during 1-2 seconds. Although if this happened frequently I would be a bit nervous about the reliability of my equipment and would have it checked.

ESKC (Uppsala/Sundbro), Sweden

Airborne_Again wrote:

I don’t know if there are any rules. It would depend on the pilot’s judgement. Personally, I wouldn’t care. Nothing much can happen during 1-2 seconds. Although if this happened frequently I would be a bit nervous about the reliability of my equipment and would have it checked.

This is what I was looking for. Thanks.

RobertL18C wrote:

Possibly a loose antenna connection? Shouldn’t happen and the ground stations are monitored by ATC, so probably worth checking. The fact it happens on both VOR and LOC might suggest antenna?

Was thinking the same. I’ll have the shop check at next visit.

EHRD, Netherlands

It depends how high? and how long you need guidance for?
- If GS is lost above 1000ft gate, no reason why not down to LOC minima or some MDH?
- If GS is lost bellow DH+500ft, I will personally go missed
- If NAV is lost while I am on a good heading, I would wait few seconds
- If NAV is lost while I am chasing the needles, I will go missed

On AP you don’t have much choice other than hand flying or go-around, it will disconnect and may or may not hook again and takes some mental effort to hook it again, something I am happy to fiddle with in VMC but I would not want to try to debug a legacy autopilot in IMC, it’s already a lot of hassle to keep an eye on it

Last Edited by Ibra at 29 Aug 18:45
Paris/Essex, France/UK, United Kingdom
35 Posts
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