|
|
Rick writes:
"... some EFIS systems (especially the inexpensive ones) derive
their attitude depiction data from GPS and pitot data. In other words,
if you lose GPS or your pitot (blockage, icing, etc.), your ATTITUDE
depiction will become INACCURATE. <snip> The most startling thing
that I found out during my research is that the FAA CERTIFIED, Chelton
system does have the problem of inaccurate attitude data with the lost
of pitot input. Their manuals state that the attitude indication can be
off as much as 2.5 degrees when pitot data is not available....>>>
2.5 degrees ??!!??!! Horror of horrors!
Rick's post proves two things. One is that a little knowledge is a
dangerous thing. The other is that if you start with a conclusion you
can find facts that "support" said conclusion.
Now for some real facts and reason. Most mechanical AH gyroscopic
instruments rely on the gravity to determine which way is up. The
influence of this "gravity vector erection" is small compared to the
forces that maintain the gyroscope "rigid in space". Imagine a
pendulum hanging in a jar of honey. If you roll the jar, the pendulum
will point in a new direction for a while but will, over time, return
to pointing straight down. Likewise, if you roll into a coordinated
turn with a mechanical gyro AH and hold the turn for several minutes
the AH will eventually show level flight when you are still turning.
Your inner ear also has this property, but for different reasons.
When your are in a turn the centripetal force vector and the gravity
force vector combine to create an apparent gravity vector, which is not
vertical (with respect to the earth).
Mechanical gyro AHs, and apparently GRT, rely on the assumption that,
on average, aircraft fly straight and level.
Now suppose you were a cleaver guy and you knew the velocity of the
aircraft and the rate of turn (degrees per second) you could calculate
the centripetal force vector and then subtract that from the apparent
gravity vector to get the actual gravity vector. You would know which
way is up regardless of how long you maintain the turn. Chelton uses a
proprietary algorithm to do this very thing and delivers the resulting
superior performance. The other benefit of this is that the Chelton
ADAHRS is the only low cost solid state AHRS that does not have a "kill
maneuver" that will cause significant attitude errors (e.g. low rate
climbing turn).
Will the Chelton attitude accuracy degrade with loss of air data? Yes,
but in it's degraded state it still has better performance that its
competitors over its entire operating range. Rick's assertion that his
GRT AHRS is superior to the CFS ADAHRS because it does not have an air
data "problem" is like saying a car without wheels is superior to a car
with wheels because it does not have a
potential flat tire "problem".
The bottom line is that the Chelton system has undergone extensive
certification testing and has proven its performance. All the other
wannabes can claim to be "just as good or better" but until they prove
it empirically you should not take the claims as fact.
Having spent the last 14 years designing, building and flying cockpit
flight information systems and having the experience of having
thousands of these systems deployed I can say that it is never a good
idea to fly actual IMC without mechanical backups and that most
experimental EFIS systems only work great when you don't need them,
although many fail that test as well.
Regards
Brent Regan
|
|