ADIRU is the acronym for Air Data - Inertial Reference Unit.
is a small computer that corrects pitot-static information (airspeed, altitude, vertical speed) for high altitude operations. (The progression for aispeed is Indicated - Calibrated - Equivalent - True) Flight at high altitude and high mach numbers requires corrections be made to the information coming from these probes.
has been discussed elsewhere. It is the system that uses accelerometers and ring-laser gyros to provide flight instrument platform (up and down) and navigation information. The IRU may also include GPS.
Alignment of this unit is necessary on startup because it begins with where it is now (at which gate of what airport) it then finds local vertical and measures the rotation of the earth both as a credibility check for present position and for north-location for the compass portion of the unit.
From there, we load a flight plan into the Flight Management Computer and the IRU actually measures our movement along (or away from) that flight plan. The "reference" means that it also tunes VOR navaids and measures distance from those stations as a cross-check of present position provided by the inertial part of the unit.
and IRU are combined because they have overlap functions. For example if the IRU portion knows how fast we are moving across the surface of the earth(GS
), and in what direction, and the AD
unit knows what our True Airspeed (TAS) is, the difference between heading and course made good, and the difference between TAS and GS
is the wind at our altitude.
This wind is displayed for us, even though the autopilot will track the required course regardless of wind.
Happiness is not seeing another trite Ste. Maarten photo all week long.