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Barometric Reference Selection Module (AP9112)

Product Overview

The AP9112 Barometric Reference Selection Module is a 28VDC powered switching unit developed to provide an interface between an Air Data Computer and a Honeywell CDSR as part of a VIP fit on a Eurocopter Dauphin.

The AP9112 alows the pilot to switch between a manually selected QFE / QNH and a preset barometric reference of 1013.25 mbar using the "STD" selection push button on the CDSR console.

The AP9112 module is installed in line between the barometric setting potentiometer and the Air Data Computer and allows the Air Data Computer input to be switched between the Aircraft potentiometer and an internal precision potential divider network, with a ratio equivilent to 1013.25 mabr (29.9121 inHg).

Source selection is achieved via an opto-isolated discrete input, which may be configured for momentary or latched operation.

Potentiometer substitution is achieved using a high reliability, low contact resistance sealed relay, with the Aircraft potentiometer wired to the normally closed contacts, thus providing the additional safeguard of reverting to the manually selected input in a power fail or error condition.

The AP9112 has a deterministic mode of operation using simple combinational logic and contains no software or complex hardware sub-systems.

Product Features

Conversion Law

As standard the AP9112 provides a substituted barometric ratio compliant with the ARINC-575 specification,
as follows, however other ratio options can be easily accomodated.

Ratio = ( - 0.34224 X inHg ) + 10.61328

The interface is fully specified in Nginuity Interface Control Document AP9112-ICD, available from our downloads page (login required).

Terms Explained


The barometric altimeter setting which will cause the altimeter to read airfield elevation when on the airfield.
In ISA temperature conditions the altimeter will read altitude above mean sea level in the vicinity of the airfield.


The barometric altimeter setting which will cause an altimeter to read zero when at the reference datum of a particular airfield (generally a runway threshold). In ISA temperature conditions the altimeter will read height above the datum in the vicinity of the airfield.

QFE and QNH are arbitrary Q codes rather than abbreviations, but the mnemonics "Nautical Height" (for QNH) and "Field Elevation" (for QFE) are often used by pilots to distinguish them.


Average sea-level pressure is 101.325 kPa (1013.25 mbar) or 29.921 inches of mercury (inHg) or
760 millimeters (mmHg).