I believe you are confusing the original question regarding the FAA exam
questions on primary/secondary instruments. This is not a Boeing 777 with
primary/backup flight control computers and instruments with differing
specified requirements for MTBF. In that context *all* flight instruments in a
typical GA (as tested on the FAA IR written) are "primary instruments." The
primary/supporting questions refer to which information is most relevent to the pilot
during a particular flight manuver or attitude.

Tarver Engineering wrote:
: Fact, but nonsequitor.

: Not exactly. In fact, my reason is why FAA tends to insist on certain
: equipments for an approach.

Quite correct... where "equipment" can be (but not limited to), VOR, DME, LOC,
GS, GPS, Loran, etc... that could be construed as having "primary/secondary"
functionality. For example, "Hrm... NAV1 seems to have died... let's use NAV2." The
*FLIGHT INSTRUMENTS* are required for IFR flight... i.e. altimeter, rate-of-turn,
airspeed, etc.


: He's not using primary in the same way you are.
: In your context, all of the instruments are "primary flight instruments."

: No, each instrument system has it's own level of certification and
: acceptable MTBF.

Also true. Also irrelevent.

: In the context of the FAA pedantry for the instrument knowledge test,
: those instruments are divided into "primary" and "supporting" role for
: each flight regime they list. What is a primary instrument in one
: regime
: is a supporting in others.

: The secondary instrument gives the operator a cross check capability and may
: be of a lower reliability.

Almost true. WRT your primary/secondary equipment argument (think NAV1/NAV2),
this may be the case. WRT FAA's definition of "primary/supporting" flight
instruments, not so much. Cross-check: maybe. Lower reliability: perhaps. Slightly
different information that can be interpretted to obtain equivalent information to the
primary instrument: absolutely.... that's the point.

-Cory
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