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Old 30th Aug 2007, 11:13
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Question PEC

Preparing for my helicopter IR and reading The Air Pilot's manual Vol. 5 (Trevor Thom). near the end he talks about a position error correction of 50' to be added when calculating minima. Only for a precision approach.

Where does this come from? Is it a UK requirement?

Other schools I have flown with outside the UK do not have this and use 200' (ILS) straight off the Jeppesen plates even for light aircraft, like JAR-OPS.

What minima do private pilots and GA pilots in the UK use? JAR-OPS or must they always calcualte their own like it shows in this manual using OCH and procedure minima?
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Old 30th Aug 2007, 18:48
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You've got two questions here.
1. do people calculate minima from OCA/H etc or just use the figures off Jepps or equivalent? - everyone uses the figures off the plate since when you look in the relevant section you find that the figures on the plate are calculated exactly as you need to.
2. Do people add PEC to precision approach minima. Most light aircraft pilots don't, usually from sheer ignorance of what it's all about. In the heavier world, I can't remember the last time I flew an aircraft without ADC's that made the PEC = 0, so the problem goes away.
As for why it's there, it goes back to the amount of obstacle clearance you get during the approach and missed approach from PANS-OPS (so therefore it applies in most of the world, not just the UK). On a non-precision approach you have 250 feet (or more) clearance above obstacles so the PEC of 30-50 feet on most light aircraft is pretty academic. However, on a precision approach the clearance can be a lot less (theoretically down to the same 35ft you get on take off) so all of a sudden the height/altitude at which you start the go-around becomes much more critical.
Of course the problem is that there are always anomalies that make the system look silly, like Biggin Hill where the DA for the ILS if you add 50ft ends up higher than the MDA for the Localiser only approach
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Old 30th Aug 2007, 19:26
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Where does this come from?
Your altimeter is a measuring instrument, which may have a systematic error that depends on conditions, often related to the siting of the static ports. Some airframe/altimeter combinations have zero PEC at approach speeds, others a few tens of feet in one direction or the other. 50 feet is a fairly conservative estimate, and should only be used if your POH/AFM doesn't offer a figure.

What minima do private pilots and GA pilots in the UK use? JAR-OPS or must they always calcualte their own like it shows in this manual using OCH and procedure minima?
Without checking in detail, I don't believe there's a difference between the requirements of the UK AIP (the legal requirement for non-public transport flights) and JAR-OPS1.

As BizJetJock suggests, it's a bit daft to add PEC for a precision approach but not for a non-precision approach, though that is the advice. In practice, for single pilot ops, while RVR is known to everyone, the altimeter reading at which you make a decision to land or go around is effectively a matter that remains between you and your priest.
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Old 31st Aug 2007, 05:12
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Thank you for the replies, it makes a lot more sense now. Maybe the manual was talking about calculating minima for the Uk IMC rating.
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Old 31st Aug 2007, 11:17
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I think you misread my post Bookworm; I was actually trying to point out that the obstacle clearance on a precision approach can be so small that the altimeter error does become significant. Just because there are occasions when it isn't doesn't invalidate the principle.
Sadly also your comments about the altimeter reading for a go-around decision reflect a common ignorance about the margins being operated to. I used to find when teaching IR's for a living that an hour showing students the section of PANS-OPS dealing with obstacle clearance usually resulted in them wanting to go around several hundred feet higher
Perhaps you ought to do some reading.
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Old 31st Aug 2007, 12:47
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I think you misread my post Bookworm
You're right, I did. I should have pointed out how wrong you were, not complimented you on the excellent point.

I was actually trying to point out that the obstacle clearance on a precision approach can be so small that the altimeter error does become significant. Just because there are occasions when it isn't doesn't invalidate the principle.
The chosen DH on a precision approach is not your guarantee of obstacle clearance -- your adherence to the glideslope is. If you remain on the glideslope a late decision does not compromise obstacle clearance. By contrast, on an NPA, your MDH is directly connected to your obstacle clearance.

It is utterly daft to ignore a known and predictable altimeter error of the order of 50 ft for a NPA where obstacle clearance might be as little as 246 ft.
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Old 31st Aug 2007, 15:28
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Mmmm, never been anywhere with obstacles in the missed approach area then , have you?.....
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Old 31st Aug 2007, 16:05
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The missed approach phase includes an initial height loss margin of between 130 and 160 ft depending on aircraft category.

But to answer your question, I've not been anywhere with obstacles in the missed approach area in an aircraft with a climb gradient of anything approaching the dismal 2.5% on which the missed approach protection is based.

For both of those reasons I'll stick to my belief that altimeter error is likely to be considerably more critical for the final segment of a non-precision approach than on the missed approach segment of a precision approach.
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