MGL Avionics Stratomaster Ultra Horizon XL Owner’s manual
Page 64
So, the formula to use became:
P=P_0*(1-6.8755856*10^6*H)^5.2558797
Where:
P= pressure
P_0= pressure at sea level (1013.25 mb on the “standard day”)
H= height in feet above mean sea level
This formula is used to a level of 36.000 ft (10 975 m) . Above that a different formula is
used.
Now what does all of this mean to you?
It means you have an exceedingly accurate altimeter that implements the ANSI standard in
detail. It does not mean that your altitude is correct as indicated !
Your local pressure is determined by many factors, temperature and weather are only a few.
All of these influence your altitude reading. Place your Stratomaster Ultra on the ground and
run it for a whole day. Notice slight changes in the altitude (perhaps up to 60 ft (18 m) or
even more?). These are caused by local changes of atmospheric pressure.
These are some of the reasons the altimeter has a QNH setting. This is simply a correction
factor that is entered into your altimeter to correct for current local atmospheric conditions. As
long as all altimeters use the same QNH they should all read the same altitude. This is all
that aviation requires.
Should you be interested in your actual altitude, we recommend using a GPS. It is probably
the most accurate method available today. However, it is not suitable for use on an aircraft
due to the inability to display altitude according to ANSI standards and local QNH settings. It
is important that all aircraft fly to a common altitude reference,
even if is not correct
.
True airspeed (TAS)
The Stratomaster Ultra instrument can indicate true airspeed. TAS is used for most internal
calculations where air distance is of importance.
What is TAS and how is it calculated?
TAS is indicated airspeed (ASI) compensated for altitude and temperature. Often pilots
ignore the effect of temperature and only take altitude into account when converting ASI to
TAS. For practical purposes this is quite accurate and gives a good reflection on your true
airspeed. Keeping in mind that ASI measurement is subject to errors caused by airflow
around your aircraft, there seems little point in taking this calculation to absolute resolution.
Again, we have decided to use a formula often used by pilots. This way the instrument
reading will agree with what pilots are used to.
Based on Worthingtons 13
th
edition page 349:
Add 1.75% of IAS per 1 000 ft (304.9 m) increase in altitude above sea level.
We assume here that IAS = RAS (rectified air speed).
The Stratomaster Ultra applies the above rule but works it at a finer resolution of 100 ft
(30.5m).
