Garmin G1000 Mooney M20TN - Manual (Page 277)

190-00647-00 Rev. A Garmin G1000 Pilot’s Guide for the Mooney M20M/M20R/M20TN 6-41
HAZARD AVOIDANCE
TAWS utilizes terrain and obstacle databases that are referenced to mean sea level (MSL). Using the GPS
position and GPS-MSL altitude, TAWS displays a 2D picture of the surrounding terrain and obstacles relative
to the position and altitude of the aircraft. Furthermore, the GPS position and GPS-MSL altitude are used
to calculate and ‘predict’ the aircraft’s flight path in relation to the surrounding terrain and obstacles. In this
manner, the TAWS system can provide advanced alerts of predicted dangerous terrain conditions. Detailed alert
modes are described later in this section.
BARO-CORRECTED ALTITUDE VERSUS GPS-MSL ALTITUDE
Baro-corrected altitude (or indicated altitude) is derived by adjusting the altimeter setting for the local
atmospheric conditions. The most accurate baro-corrected altitude can be achieved by frequently updating
the altimeter setting to the nearest reporting station along the flight path. However, because actual atmosphere
conditions seldom match the standard conditions defined by the International Standard Atmosphere (ISA)
model (where pressure, temperature, and lapse rates have fixed values), it is common for the baro-corrected
altitude (as read from the altimeter) to differ from the GPS-MSL altitude. This variation results in the aircraft’s
true altitude differing from the baro-corrected altitude.
PREMATURE DESCENT ALERTING (PDA) USING GPS-MSL ALTITUDE
The Premature Descent Alert (PDA) is annunciated when the aircraft is significantly below the recommended
minimum altitude for an approach. The aircraft’s GPS-MSL altitude is compared to a calculated above-
ground-level (AGL) altitude along the estimated path to the airport. When the aircraft descends below the
calculated minimum altitude for the estimated path, the PDA alert is annunciated.
Non-standard atmospheric conditions may result in differences between GPS-MSL altitude and baro-
corrected altitude (which is normally referenced when flying). These differences may reach a level that can
trigger a PDA when flying the published minimum altitude as indicated on the altimeter.
Operations in below-standard-temperature environments may require the pilot to compensate for the non-
standard temperature by increasing the minimum altitude that is defined in the approach procedure for
standard atmospheric conditions. Differences between GPS-MSL altitude and baro-corrected altitude typically
increase as height above ground level increases due to non-standard atmospheric effects. In response to PDAs,
the pilot should always ascertain the appropriate altitude is being flown in consideration of atmospheric
conditions.
For more information on PDA alerting, see the section on TAWS alerts.