Altimeter Checks

[OZBorn]

With reference to ENR 1.5 and the issue of Altimeter Checks for an ILS. AIP states:

"If the check indicates an unexplained discrepancy, the ILS should be discontinued."

My question is, how much is considered to be abnormal? Assuming of course you apply common sense and say, "well I'm half a dot high therefore I expect the altimeter wont read exactly the altimeter reading as marked on the plate" I'm aware of the effect of very low temperature and the old "if you're low you're good to go" adage but just saying an unexplained discrepancy is a little vague an answer to give to my IRE.

[QSK?]

Wasn't this discussed in another thread about 4 months ago?

[Richo]

Hi Ozzie

The intention of the procedure is "to confirm the accuracy of the alitimeter (in use)" as most ILS's are bringing you to within 200ft of the ground, I would suppose that this is a desirable thing.

Yes unexplained discrepancy is a bit vauge. It could be any discrepancy up to but not outside the IFR altimeter serviability of 60ft. In cases of two Altimeters it could also be 40 ft either side ECT.

It is really up to you to decide what is UNEXPLAINED.

As for the "OFF SLOPE" bit, I could get all stuffy and say, you should not be off the slope, but I would be bieng very hypocritical, I suppose.
One operator I worked for had a description in thier ops manual to assist in making the decision. I am going from memory so take this information as not neccisearily correct!!

1 dot = 20 ft at 1 nm

Therefore if your check Ht distance is 5.4DME 1 dot = 108 ft
If I can find the accurate reference I will post it, BUT once again this info may not be accurate.

richo

[Muffinman]

Yep, happy to back up richo :Dwith the same rule of thumb that we use at my abode and also where I use'd to teach.


If -
@ 6nm from the threshold : 1 dot +/- slope = 100 ft

Each nm either side of the 6 nm position, 20 ft can be +/- from the deviation.

ie @3 nm from threshold -: 1 dot deviation = 40 ft (100 - 60).

ie @9 nm from threshold -: 1 dot deviation = 160 ft (100 + 60).

Hope this helps

PS dunno if it's true - might have to wander off the glideslope one day:E :eek:

[OZBorn]

Thanks Richo and Muffinman. Much appreciated.

[Hailstop3]

Don't want to nit pick but as far as i know, most DME are at the VOR position arent they, which is not at the threshold, so that wouldnt be super accurate would it? Would give a reasonable indications though id imagine.

Yep, happy to back up richo with the same rule of thumb that we use at my abode and also where I use'd to teach.


If -
@ 6nm from the threshold : 1 dot +/- slope = 100 ft

Each nm either side of the 6 nm position, 20 ft can be +/- from the deviation.

ie @3 nm from threshold -: 1 dot deviation = 40 ft (100 - 60).

ie @9 nm from threshold -: 1 dot deviation = 160 ft (100 + 60).

Hope this helps

PS dunno if it's true - might have to wander off the glideslope one day

[novicef]

Where is the +_ 60 ft and discrepancy of 40 ft between 2 altimeters written down? I read somewhere that for RVSM operations the discrepancy was 70 ft.

[Atlas Shrugged]

If I can find the accurate reference I will post it, BUT once again this info may not be accurate.


:D:D:D




.

[Swanie]

I don't have the reference here but I remember the altimeter error of 60' be extended upto 75' (or something close) provided;
-you have 2 altimeters
-you check it again at the next possible chance

If I remembered correctly, then is it safe to say that 75' would be the maximum "discrepency" your allowed. As the altimeter would be U/S otherwise..:confused:



Besides if your off-slope, then you wouldn't be expecting the correct check height in the first place, therefore it's not an unexplained discrepancy:eek::ok:
(just fix it)

[Muffinman]

As I said, it's a rule of thumb, and the DME position is irrelevant.

Think about it.

[Hailstop3]

As I said, it's a rule of thumb, and the DME position is irrelevant.

Think about it.

Well i have thought about it, and thats why i asked. Please enlighten me to why it is irrelevant.

[UnderneathTheRadar]

Why? Because the rule is "Distance from the threshold" - not DME.

Check your DAPS, the OM position is given with a "Distance from the threshold" (as for YMEN) or for places without an OM (say YMAV) then DME/Distance from threshold conversions are given.

Good rule - now I'm going to sit down and do the maths to prove it!

UTR.

[Hailstop3]

I understand now, and i remember that now. Thanks for clearing that up :ok:

[Muffinman]

Back home - thanks UTR - ah.... is that two I can chalk up now marcuste747? ;)