> First, "IFR" is "Instrument Flight Reference."

IFR is Instrument Flight Rules, referring to specific
portions of the Federal Aviation Regulations (FARs).  It
includes specific requirements to report position to the
ground controllers.  Reporting requirements can be
waived if being monitored on radar.


> This is only flown when visibility is low, such as when you
> are in clouds.

There are different rules for different types of operations.
Private usage is FAR part 91; commercial (for-hire) is part
135; and airliners (scheduled commercial service) is part 121.
The same aicraft may be operated under different parts
depending on the purpose of the flight.


> Controllers often assist in IFR conditions, but by no means
> are airplanes under IFR conditions

Frequently "IFR conditions" is used to denote an aircraft
being operated solely by reference to instruments due to
weather.  A more correct term is "instrument conditions"
as that has no ambiguity with rules versus weather.


> just because they are commercial

Certain commercial operations require operating under IFR
flight plans at all times.  If not by federal mandate, then
by company policy.


> or because they are in controlled airspace.

Basically true.


> In fact, you really want to avoid IFR whenever you can.

If you can -- both the rules & the weather conditions. :-)


> "VFR" is what you fly on clear days - "Visual Flight Reference".

VFR is Visual Flight Rules in the FARs.  And if you are below a
certain altitude (18,000 is the base of class A airspace, formerly
called positive control area (which ends at 60,000 feet)).


> It's perfectly alright to be dead silent for minutes or hours
> at a time in these areas.

VFR or IFR, it's quite common to not converse wtih a controller
for long periods in unpopulated areas.  The northeast US is so
heavily populated, there are special routes and flow controls.
And deviation from your route will be noticed fairly quickly --
mainly because it impacts airspace that the controller wants to
put some other aircraft into.


> Finally an "off" transponder is not suspicious. First, they get
> turned off on accident all the time, or they fail, so that is
> the first assumption.

Transponders are on the minimum equipment list (MEL) of large
turbine powered aircraft.  I believe that they are usually
installed in airliners in pairs for redundancy.  A failed
transponder that keeps a multi-million dollar aircraft on the
ground is not acceptable business practice.


> Second, they need to be off for a while before the system
> really comes to the conclusion that the transponder is no
> longer squawking. It's not like the radar has a constant link
> with the transponder, so it being turned off is not instantly
> apparent to those on the ground.

It's not constant, but the radar sweeps around every 6-10
seconds.  Lack of all returns for a minute is probably enough.
And if the aircraft has a data block assigned by the computer,
lack of returns will trigger an alert to the controller.

                                                Lee Jones

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