I know where it comes from.
AIP AD 1.1-6 2.6.1
When calculating Decision Height (DH), account must be taken of the errors of indicated height which occur when the aircraft is in the approach configuration. Details of the Pressure Error Correction (PEC) should be available from the aircraft Flight Manual or handbook. In the absence of this information a PEC of +50 ft has been found to be suitable for a wide range of light aircraft and should be used. This addition of 50 ft need only be applied to DH. The required RVR should be calculated prior to applying the PEC.
If there's a calibration error between indicated altitude and the calibrated altitude, it would seem to make sense to use it. Daft to fly down to indicated 200 ft knowing it is actually 150 ft.
The irony is surely that any altimeter error is much more critical on an NPA than on a PA, yet the UK CAA policy is (was?) to apply it only to a PA.
Some teach a concept of personal minimums where the pilot establishes their comfort zone, but this usually applies to conditions in which a pilot is willing to fly. In the US, there are no suggestions to add any amount to the DA on a precision or vertically guided approach for non commercial operations.
The altimeter and static system must be checked every 24 months to conduct IFR flight. Also, there is a correlation check required of any encoding altimeter with the primary baro altimeter indication. The altimeter is checked on every flight when the local altimeter setting is entered and compared against field elevation. While in flight, ATC continuously monitors altitude indications and any error over 200 feet requires that one turn off the mode C operation of the transponder. On a precision approach, we are taught to cross check the crossing altitude at the FAF on each approach to provide a sanity check of the GS indication and the altimeter - altimeter setting indication. In other words, there is a good amount of cross checking the altimeter system on a regular basis.
All this said, the minimums provide safe operation for a proficient pilot and I don't see the need under normal circumstances to adjust the DA. I am more conservative with a non precision approach to a MDA and will add 50 feet to the MDA as my level off altitude, leave a little up trim pressure, so that any attention lapse will tend me to gain altitude, and if necessary work my way gingerly down to the MDA as there is no provision for sink thru as there is on a vertically guided approach with a DA.
PEC will be listed in the Flight Manual if it has been measured. You apply whatever is listed in the manual. If your aircraft is uncalibrated i.e it is not part of the certification, it is normal to add 50 ft PEC to be on the safe side. It is only applied to precision approaches.
What's the problem with AIP plates (apart from them usually not listing DA/DH, but OCA/OCH and TDZ elevation)?
The AIP plates mostly show the OCA not the MDA, so you have to use a formula to do the conversion.
A lot of pilots don't know this and think OCA=MDA.
Also most AIP plates are barely readable in A5 size.
I am unable to find a legal document giving much information about it at present.
The basis for all this is ICAO Doc8168 PANS-OPS which again forms part of all national and multi-national (JAR-OPS, EASA-OPS, EU-OPS) regulations. (Start your reading in Part I, Section 4, Chapter 1).
If I fly privately, I will happily use the minima on the Jeppesen plates (not AIP!).
What's the problem with AIP plates (apart from them usually not listing DA/DH, but OCA/OCH and TDZ elevation)?
Every measuring system will have an error.
That's correct. An ADC is just nothing but a little box that takes analog pressure and temperature readings, converts them to digital bit patterns, does some (pre-)processing with them and passes them on to whatever device you use for further processing or displaying. Every single source of error that is present in a purely analog installation is also there when an ADC is used, like positioning errors of the pitot tubes and static ports, blockage/icing of pitot, static and temperature probes and so on.
The best altitude value will always be a computed value from a weighted average that uses all available data sources like ADC output, GPS altitude and radar altimeter (still essential for autolands for example where the update-rate of GPS systems is insufficient).
what next,
My company operating manual gives us a system minima for each type of approach which in this case is equal to the EU OPS system minima. The question of PEC comes from the ops manual also telling us to add 50ft for Pressure Error Correction to a precision approach (which I suspect can be read as ILS since we aren't equipped for MLS or LPV and PAR is not exactly common). Of course we operate in accordance with the ops manual but I wondered about the actual legal requirements of adding this figure as I have seen it mentioned before, as has Peter I believe. I am unable to find a legal document giving much information about it at present.
Peter,
I was indeed aware it would not be perfect. I did however wonder if it reduced it to a NEGLIGIBLE amount that was small enough to be effectively 0. I'm going about this a slightly backwards way really, as I am trying to prove/disprove someone's comment about not having to add PEC due to having an ADC by working out whether or not it actually makes sense, as opposed to trying to find a legal reference for it because my eyes have somewhat dried up from trying to find anything about it.
because the ADCs will reduce the error to 0"
I don't think so.
Every measuring system will have an error.
In an ADC you still have a pressure altitude encoder - probably something like this (note the +/-50ft error in that spec) and there will be a microcontroller inside which gets the OAT (more errors there...) and corrects the IAS (more errors there...) into TAS using the pressure altitude and temperature. It also gets the heading from the aircraft slaved compass system (more errors there, especially if the heading comes as sin/cos or x/y/z/400Hz rather than ARINC429 digital, but anything generating an ARINC429 digital heading will still be encoding the heading from the analog synchro signals) and using the GPS GS and GPS track works out the wind, but you get big errors in that for winds which are nearly aligned with your track if there is a slight heading error...
A popular airdata computer is the ADC200 which on page 12 of the PDF shows the altitude error.
The only chance of a "perfect" altitude is GPS, with EGNOS
