Required IFR equipment

The basic instruments required for day VFR are simple: an airspeed indicator, an altimeter, and a magnetic direction indicator. Night VFR adds lighting and a few more items. IFR adds a further, more demanding layer on top of both, because now the pilot may have no outside visual reference at all.

Operation under IFR requires a gyroscopic rate-of-turn indicator, a slip-skid indicator, a sensitive altimeter adjustable for barometric pressure, a clock displaying hours, minutes, and seconds with a sweep-second pointer or digital display, a gyroscopic pitch-and-bank indicator (the attitude indicator), and a gyroscopic direction indicator (a heading indicator or equivalent). None of these are optional extras; each is there because, without outside references, the pilot has nothing else to fly the airplane by.

Beyond the flight instruments themselves, paragraph (d) adds electrical and communication/navigation items. Many instrument students memorize the full list with the mnemonic GRABCARD, which reorders the items into something easier to recall than the order they appear in the regulation.

The requirement for two-way communication and navigation equipment is written broadly on purpose: it demands the ability to fly the route filed and execute the procedures it requires, not a specific brand or generation of avionics. That wording is exactly why an airplane whose only navigation source is an IFR-approved GPS can be perfectly legal for IFR, even though the same requirement was written decades before satellite navigation existed.

Not every GPS receiver is legal for IFR navigation. Equipment approved only for VFR situational awareness -- including handheld units and tablets, however good their moving map looks -- cannot be used as the principal means of IFR navigation, for instrument approaches, or as a primary flight reference. A receiver has to be certified to the appropriate technical standard and installed under its own supplement to the aircraft flight manual.

The real question a certified receiver answers is not just where you are, but whether that position can be trusted. Receivers without satellite-based augmentation rely on receiver autonomous integrity monitoring, RAIM: the receiver checks its own solution against the satellites in view and flags when it cannot guarantee accuracy for the current phase of flight. RAIM needs a minimum of five satellites in view, or four plus a barometric altitude input, just to detect a problem, and six (or five with baro-aiding) to isolate and remove a bad satellite from the solution. Losing RAIM without an operating alternate means of navigation forces a delay, a reroute, or a cancellation.

WAAS receivers add corrections from geostationary satellites plus their own integrity message, which removes the RAIM-prediction step and opens up approach minimums with vertical guidance (LPV and LNAV/VNAV) that non-WAAS GPS cannot fly.

Aircraft with an approved minimum equipment list (MEL) follow that document when something fails: it states exactly what may be inoperative, under what conditions, and for how long. Most general aviation airplanes do not carry an MEL and instead work from the equipment lists in the type certificate, the requirements of 91.205 for the flight being flown, and any applicable airworthiness directives.

Whichever path applies, the practical steps look the same: confirm the failed item is not on any of those required-equipment lists for the flight planned, remove or deactivate it and placard it clearly as inoperative, and have the airplane's continued airworthiness confirmed and recorded before flight. A flight that is technically legal with one fewer instrument is not automatically a wise one to fly into weather.

The instrument ACS asks about flight control systems among the aircraft systems related to instrument flight, which surprises pilots who expect the list to be about vacuum pumps and alternators. The reason is feel: on instruments, the control system is the only channel through which the airplane tells you anything before the instruments do.

The Pilot's Handbook of Aeronautical Knowledge notes that flight control systems and their characteristics vary greatly with the type of aircraft. The most basic designs are mechanical and date back to early aircraft, working through rods, cables, pulleys and sometimes chains to transmit forces from the flight deck controls to the control surfaces - and they are still used today in small general aviation and sport category aircraft, where aerodynamic forces are not excessive.

That directness is what a light-airplane instrument pilot flies with. Control pressure varies with airspeed, so the airplane signals a speed change through the yoke before the airspeed indicator has moved far - which is exactly why the handbooks insist on fingertip control and on trimming precisely. A pilot gripping the controls has switched that channel off.

The instrument-relevant consequences are the trim system, which relieves the pressure so the scan can continue; the secondary controls - flaps and trim tabs - whose deployment changes pitch attitude and therefore the picture on the attitude indicator; and any powered or boosted system, where the feel a pilot has learned to read is manufactured rather than aerodynamic.