Here is a must read article, for us IFR pilots, just published in Kitplanes and was written by my friend Keith Thomassen. He also teaches classes for REALLY using the 530/430's, 480, Chelton EFIS. Here is his web address for the classes if interested;

I'm just installing a 480 and have the EFIS so I will be taking the class soon.

Jim Hergert

L4P N6XE, 350 hrs

 

 

What is required for navigating in the IFR system in an experimental
aircraft?  This question stimulates lots of opinion and much
disagreement, even if you’re asking FAA representatives.  That’s
because there is room for “interpretation” in the rules.  What are
those rules and how are they to be interpreted?  Let’s reason together
starting with what is written.

The rules for flying IFR are given in Part 91.205 for “powered civil
aircraft with standard category US airworthiness certificates”.  These
are not just the rules, they are in fact permission to fly IFR if you
satisfy them (and are rated and current).  So they are both necessary
and sufficient conditions for IFR flight in certified aircraft.

For experimental aircraft your Operating Limitations, issued after the
FAA inspection and licensing of your aircraft, determine whether you
can fly at night, do aerobatics, or use the aircraft in the IFR system.
  For IFR approval there will be a statement in those Limitations making
Part 91.205 applicable to you.

The requirements of Part 91.205 can be lumped into two groups; 1) a
list of instruments that include those for day VFR, night, and
additions for IFR, and 2) the statement that you must have “two-way
radio communications system and navigational equipment appropriate to
the ground facilities to be used”.

Most of the instruments are self-explanatory, like airspeed, compass,
altimeter, clock, etc., but there is some confusion on gyro instruments
(rate-of-turn, pitch and bank, directional indicator) now that solid
state AHRS devices are being used.  The Experimental Aircraft
Association (EAA) has worked with the FAA Small Airplane Directorate to
resolve that issue; if it performs the function it is acceptable.

A more confusing issue that is addressed here concerns GPS units for
IFR flight.  There are several FAA documents on using GPS, such as the
navigation chapter in the AIM or the circular on GPS (AC20-138a).  But
advisory circulars are just that, and are not regulatory.  So let’s
explore the use of GPS for IFR flight.

First, as the EAA also concludes, the equipment does not have to be
certified.  But they concluded in their written summary that the
required navigational equipment statement in 91.205 says that you can’t
use GPS for your primary navigation system because it is not
ground-based.  They conclude that your primary system must be ground
based (VOR’s), but that is also true if you installed a certified GPS
under TSO 129c, so more discussion is needed there.  Also, a GPS
certified under TSO 146 does allow a GPS to be your primary navigation
system.  Let me clarify these points in some detail.

If you had a Garmin 1000 (or a King KLN94, or Garmin 430/530) it cannot
be used for your primary navigation system.  These are all certified
under TSO 129c, and as such they are certified for supplemental
navigation.  That’s because the FAA has integrity criteria for
discontinuing their use in IFR flight, and the requirement for an
integrity monitoring system that can warn you not to use the GPS for
navigation.  The Receiver Autonomous Integrity Measurement (RAIM)
system must warn you if you don’t meet the RAIM requirements for your
phase of flight – 2 nm while enroute, 1 nm in terminal areas (within 30
nm of your departing or arrival airport, where you might do a SID,
STAR, or missed approach), and 0.3 nm in the approach phase (final to
missed).

Clearly, the reasoning goes, if it might be rendered unusable it can
only be certified for supplemental navigation.  That’s why such a
receiver cannot be used at an alternate airport where there is no other
type of approach (than GPS).  If there is one, of course, you can do
the GPS approach there.

Supplemental use is not all that restrictive in that you don’t have to
be navigating by VOR and DME if your GPS is within RAIM limits.  You
must, however, be able to make that flight with the GPS turned off. 
All ground navaids must be operational (don’t file a route over an
inoperative VOR), and your VOR receivers must be operational.  Of
course, you wouldn’t file GPS-Direct over routes that require RNAV
equipment (long distances in areas of sparse navaids), unless you have
RNAV equipment.  While this is commonly done, ask yourself what you’d
do if you had a RAIM failure on that segment.

So, how about your non-certified GPS?  If it has the functional
requirements of the certified equipment, you should (I say) be able to
use it.  If questioned, the burden of proof is on you that you have met
the “navigational equipment” requirement of 91.205.  The FAA could use
FAA policy or applicable court decisions to decide otherwise, but here
things are grey.  At least you should ensure that your unit does the
integrity monitoring that is at the heart of the TSO 129c requirements
and limitations.  Then, you should use it as supplemental to your
primary system in the sense I just described.

If you believe your unit meets these standards, is it ok to do GPS (and
overlay) approaches with it?  If it contains the latest Nav data base
and it does the RAIM check internally before you execute it, I say yes.
  The only difference between enroute/terminal and approach phases is
the more stringent RAIM requirement or the latter.

But the world of aviation has evolved, and now there are receivers
certified under TSO 146, which requires using the WAAS system.  If you
never wanted to do NDB, VOR, or ILS approaches, you could fly with this
GPS and a COM transceiver for all your flying, and save the money
needed for a DME, ADF, VOR, and LOC/GS receivers.  This TSO has tougher
requirements on position accuracy that can only be met by adding WAAS
error correction to your raw GPS position solutions.

Ground stations around the U.S. receive raw GPS position solutions and
send their 3D position error (they know where they really are) to
geosynchronous satellites in the east and west.  Your WAAS enabled GPS
receives the errors from those satellites and, by interpolation using
errors at ground stations near you, adds a 3D correction.  The
WAAS-corrected solution is claimed to be accurate to a meter horizontal
and two meters vertical (best case, I suspect).

The raw GPS solutions have an accuracy affected by the dilution of
precision (DOP), which comes from poor geometry (all the satellites
lumped close together gives a lousy solution).  As in any triangulation
scheme, horizontal position is best measured if stations are on both
sides of you (on the horizon left and right).  Since vertical position
cannot be determined by satellites above and below you (the earth is in
the way) your raw vertical position is not nearly as accurate as the
horizontal.  WAAS corrections largely fix that, so WAAS is critical to
vertical operations, such as using GPS altitude for terrain avoidance
(TAWS systems) or doing GPS vertical approaches.  There are other large
errors due to the slowing down of the GPS signal through the ionosphere
and atmosphere (light slows down a factor of about 9 in water), and
these too are corrected in real time since the ground stations
experience the same extra delays.

The Chelton Flight Systems are certified under TSO 146, so they can be
used for primary navigaton.  So if you purchase the experimental
Chelton system, can it be used for primary navigation? Since the FAA
also imposes integrity monitoring on 146 units, it must give integrity
warnings as specified by that TSO.  These include measuring the
horizontal and vertical protection limits (HPL, VPL), which you will
find on the satellite page of the Garmin 480, the only other GPS
certified under TSO 146.  The certified Cheton gives the required
integrity warnings through its Free Flight GPS engine and software, so
if your experimental Chelton uses that GPS engine (this is an option on
their Pro system) those warnings are given and (I believe) this
satisfies the FAA intent.

But the Chelton is currently certified for LNAV operations only, so
LNAV/VNAV and LPV approaches are verboten.  Why?  If you are going to
track a vertical GPS course to LPV minimums, for example, an FAA
requirement in TSO 146 is to determine your position 5 times per
second, not once as in all TSO 129 receivers and the Chelton.  But
there is more.

The LNAV/VNAV and LPV approaches are called Approaches with Precision
Vertical (APV).  This means that, in software, the full scale CDI
deflection is reduced as you go down the glideslope much as both ILS
localizer and glideslope courses reduce the full scale deflection as
you proceed to the runway.  The increased sensitivity keeps you in a
smaller and smaller box, and you must abort if you can’t keep the
needles off the pegs.

So here is another set of requirements on refresh times and CDI
sensitivity.  By the way, there is also an increased sensitivity
horizontally for LPV approaches, but not for LNAV/VNAV, hence the
former have the lowest minimums and visibilities (generally).  At the
moment, the only GPS available, certified or not, that can meet these
requirements is the Garmin 480, so at the moment it is a moot point and
the real issue today is whether your WAAS GPS can be used for primary
IFR navigation.

The requirements extracted here from TSO 129c or 146 are by no means a
complete set, and it’s not clear whether you need to meet others not
listed.  There are requirements on environmental, software, data, and
manuals for example, but I believe as for gyros, the main issues are
functionality (which includes fault detection).  As the pilot and
manufacturer of your aircraft however, the burden of proof is on you to
determine if you meet the 91.205 requirements for IFR flight.

Finally, only you can decide what equipment is sufficient for your type
of flying.  Redundancy is important, and everyone will have a different
comfort level with various backup options.  But I hope this gives more
insight into using one of the many new GPS systems available to the
experimental

Keith Thomassen