DH (Decision Height) or DA (MDA Minimum Decision Altitude) are the same thing and are always indicated with the 2 values (MSL and AAL - Above Airport Elevation).
DH is the word used for
ILS or
LOC approaches
DA or
MDA is the word used for
RNAV, VOR, NDB or visual approaches.
But in some case, the word DA is used also for ILS approaches.
The word has no importance, because the FMC use only the value.
More the approach is a highest precision approach, Less is the value of the DA (or DH). When you have a vertical guidance (ILS) , you can found in the charts the word "LPV" to describe the aircraft category (Localizer performance with vertical guidance). Don't use the LPV value for RNAV appraoches.
But on all charts, you have always the two altitudes indicated. MSL and AAL
The FMC displays the word DH on the PFD for ILS or LOC approahes and it displays the word MDA for all other approaches and the XML code use this value for the lastest autopilot altitude and for unlock the ALT knob.
You must use always the AAL value for the last waypoint of all approaches (this waypoint is always the runway threshold, a waypoint type "R").
samples with charts :
Here is the lower area of the ILS 25R chart of KLAX :
In the red rectangles, this is the values MSL ( 294 feet for the ILS approach and 560 feet for the LOC approach)
But in the Freenav files, you must write the values in the blue rectangles that are AAL :
200 feet for the ILS approach
466 feet for the LOC approach
Now, KLAX, RNAV approach :
For RNAV appraoch, you must use the LNAV (horizontal guidance only) value in feet AAL. Here 475 feet.
The value AAL is always the second value and is always minus than the MSL value (except if the aiport is under the sea level, case of EHAM).
LPV value is the aircraft use in final the ILS, but in freenav files, do not use this value for RNAV approach.
Last exemple with a french chart of LFPG ILS 27R :
Value to write in the Freenav file for the runway threshold :
ILS approach : 200 feet
LOC approach : 370 feet.
As you can see, in many countries, the AAL value is between brackets.
In the most cases :
- ILS approaches have value between 100 and 200 feet
- LOC approaches have value between 200 and 500 feet
- RNAV approaches have value between 400 and 1500 feet
- VOR approaches have value between 500 and 1500 feet.
This is the only exception; In the Freenav airport data files, all the waypoints have their value in feet MSL, except the runway threshold, the last waypoint of each approach.
Other exception, in the header of the SID (file SID_data.csv) , the LNAV value, Thrust reduction altitude value and acceleration altitude value are also in feet AAL (or AGL).
Francois