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Adding PEC to DA/H

Anyone able to help with a spontaneous musing?

In our light twin operations we add a fixed PEC to the plate published DA to achieve our ILS minimums. I noticed from watching some videos of airline operations that the crew do not add any amount to the plate published DA.

Is this to do with them having more sophisticated equipment that eliminates PE (something about ADCs is in my mind here)?



United Kingdom

For us uneducated, you guys freely use acronyms that make it difficult to follow, so IMHO WTF does PEC, PE, and ADC mean?

KUZA, United States

I fly to the minima on the plate.

EGTK Oxford

NCYankee, my apologies I was not trying to be mystical!

Pressure Error Correction (thus PE being Pressure Error) and ADC being Air Data Computer.

After attempting to read up on this, I must have become a little too familiar with the acronyms!

United Kingdom

IMHO = in my humble opinion

WTF = what the ****

The PEC business has been often discussed and I don't think anybody is entirely sure where it comes from.

Normally, IIRC, it means adding 50ft to any precision approach DH.

I used to get some IR training with one very experienced jet pilot, who used to work as an examiner for a large UK FTO, and he told me the PEC was originally an FTO (=ATPL flying school; EASA is changing the name to "ATO" and getting them all to write procedure manuals) invention which improved their pass rates, and since pass rates are published, this made them look better to both the CAA and potential customers. It's easier to fly an ILS to 250ft than to 200ft.

Other folklore suggests the CAA wanted it, because most planes used in the training scene are shagged and their altimeters are a bit off, so this adds a safety margin.

When I did my initial IRT (with a CAA staff examiner) his view was that since I was flying an N-reg, which has the 2-yearly FAA altimeter checks, there was no need for this and I could fly an ILS right down to the published DH (often but not always 200ft) and then initiate the go-around.

An ADC doesn't by itself improve your altimeter accuracy. In the GA scene, any "real" altimeter is still fed from the static piping. If I installed an ADC I would have airdata (altitude, IAS, TAS, heading) fed digitally to my GPS which, knowing the GS and mag track, could now display the actual wind. This is nice to have but I wouldn't spend a few k installing something which tells me something that is blindingly obvious as soon as one gets airborne (whether I have a tailwind or headwind is obvious from the TAS-GS difference, and the TAS can be read off the altimeter subscale).

I think most (all?) glass cockpit (EFIS presentation) installations have an ADC because they have to get the altitude into a digital form somehow. But the UK IR testing scene applies the PEC to glass aircraft also.

I fly to the minima on the plate.

Shoreham EGKA, United Kingdom

JasonC and Peter,

Thank you for your views on this. I have been doing my best to read up on this and have not been having too much success.

What I was alluding to before with reference to an ADC is that a couple of non referenced points that have been brought up in the discussions I have read refer to it "not being applicable because the ADCs will reduce the error to 0". This I guess makes sense to me assuming the likes of even a G1000 installation in a C172 or something since it surely can have the correction built into it which outputs to the pilot a theoretically perfect altitude?

United Kingdom

It, like QFE, seem to be curious UK relics.

EGTK Oxford


If I fly privately, I will happily use the minima on the Jeppesen plates (not AIP!). But as I do mostly commercial flying, my "holy bible" are the operating manuals of the company I fly for. Those contain all the applicable additonals. Ours have tables of "system minima" for the different types of approaches that apply if they are higher than the Jeppesen minima. At the flying school for example it is a 400ft ILS minimum for single engine and 300ft for twin engine. At my commercial opeartion it is 250ft, even if our avionics fit is basically certified for CAT II. But there is no fixed figure that needs to be added to the Jeppesen minima (like the PEC of Pirho - never came across that acronym before, for me that is periodic error correction in a telescope mount). For example our system minimum for NDB DME approaches is 400ft which is the same as on most Jeppesen plates, so we can use the Jeppesen figures.

EDDS - Stuttgart

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

Shoreham EGKA, United Kingdom

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.


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.

United Kingdom
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