henra, #259
Is there a voting system between ADR and IR ?
I understood there is a voting system between the
ADR's and a voting system between the IR's. And that
logic was described.
But between the two? How would you vote between Air
Data and Inertial Data? Normally they are complementary
(not competing) data.
The idea of combining > 1 independent, limited reliability
sources to improve overall performance is not new. For example,
GPS / inertial mixing has been in use for some time. A brief
description may help to illustrate why this is a good idea,
with of course apologies for those who know all this already.
An ins system (IRU in airbus speak) works by dead reckoning
and is subject to drift over time. By dead reckoning, I mean
calculated from the point of origin, or start position. The
accumulated position error could be as much as several
nautical miles per hour, though current state of the art may
be much better than this. GPS doesn't work by dead reckoning,
but by direct measurement of absolute position and has far
greater accuracy, to within a few feet over much of the earths
surface. However gps can suffer from signal dropout and loss of
accuracy in some regions and is susceptable to jamming.
With the gps / inertial mixing scheme, some very smart software
augments the ins data with that of the gps to provide better
long term accuracy, redundancy and improved error detection.
For example, if the margin between the ins and gps data exceeds
a given threshold for a number of samples, one or other can be
tagged as a degraded source, while still providing good data
over the short term. That is, we now have a holdover capability
in the event of a temporary fault from either source.
By now, you can probably see where i'm taking this in terms of
a baro / inertial mixing scheme. Ins data could be used to
augment and cross check air data information. From what i've read,
the current systems are independent and any results of cross checking
between the ins and air data speeds (baro) are not presented to the
flight deck. The problem of using ins data to correlate air speed
is that the ins measures ground speed. However, at any given point
in time, there is a historical timeline and ongoing relationship
between the two. This could be used in the short term to provide
an indication of air speed in the event of a temporary pitot tube
failure. If one assumes that the wind speed remains fairly constant
short term, such a baro inertial mixing scheme could be a valid
solution to the loss of air speed data, upon which so many other
subsystems depend.
There other ways to measure air speed, such as the airbus developed
laser measuring system and which is, iirc, available as an option
on the A380. Hopefully, it will become mandatory at some stage.
Feel free to hack at all this if you like :-)...