Flight instruments are those that you use to control the aircraft without outside visual reference. They include the "Sacred Six" - the altimeter, attitude indicator, airspeed indicator, directional gyro or heading indicator, vertical speed indicator, and the turn coordinator or turn and bank indicator. Each instrument speaks its own language, and each has nuances and limitations unique unto itself. When you fly on instruments, you rely on the application of your knowledge of these nuances.
En route to your instrument rating, you'll hear the examiner ask some variation of several basic questions. For example, "Which instruments does the pitot/static system serve?" (Altimeter, airspeed indicator, vertical speed indicator.) When gremlins target an airplane, an instrument pilot's knowledge of systems can make the difference in how the flight ends. You must have an attitude that demands knowledge for the future's sake. It serves applicants well when examiners ask for diagrams of the pitot-static system, or electrical system, the vacuum (or pressure) system, and the relationship of each to the flight instruments. Airplanes are machines, and machines fail.
Every instrument examiner can recount myriad ways that flight instruments fail. When it happens, quality training is what keeps the airplane in the air. So examiners might ask questions such as, "How does the altimeter work?" and, "What are its limitations?" Because the PTS now includes metric limitations, it's possible (if unlikely) that your examiner will address metric conversion. The introduction to the PTS advises you to round to the nearest 10-meter increment, or even altitude (in hundreds of feet) as necessary.
Instrument pilot examiners have an altitude fetish, and they sprinkle questions about it throughout your testing on the flight instruments, navigation charts, approach charts, and wherever else they can find a niche they can fit an altitude question into. We pummel applicants with queries such as, "Tell me about the five types of altitude." Most applicants understand indicated altitude because it is what a properly set altimeter shows. Applicants explain pressure and density altitude less clearly sometimes. Finally, some applicants confuse true altitude (the height above sea level) with absolute altitude (height above the terrain). Pages 3 and 4 of the Pilot's Handbook of Aeronautical Knowledge, (Advisory Circular 61-23C), discuss these succinctly and well.
It doesn't end with altitude, either. Examiners also focus on airspeed. Our airspeed questions might include, "Tell me about the airspeed indicator, and the different types of airspeed." You should understand three kinds of airspeed - indicated, calibrated, and true. You should also be able to answer the follow-up question, "What are the limitations of vertical-speed indicators?"
The examiner wants to know whether you understand that most altimeters and vertical speed indicators do not give instantaneous indications, and that abrupt maneuvering, turbulence, and icing can affect the instrument.
The last item leads to another inquiry - "Discuss icing scenarios." You should be familiar with all the references listed in the PTS. For the instrument test, you would do well to read Chapter 10, "Icing," in Aviation Weather (AC 00-6). Icing is a cumulative hazard and is among aviation's major concerns. All clouds at subfreezing temperatures have icing potential, but ice may not form even though the potential exists. Your examiner will want to be sure that, when it does, you know how it affects the flight instruments and how to respond. We want you to know not only what systems ice attacks, but what systems remain operational.
This leads to questions like, "The attitude indicator works from what system?" This sends applicants back to the pitot/static diagram some examiners ask applicants to create. Some airplanes use pressure to drive gyroscopic instruments, but most have vacuum systems. Modern vacuum systems commonly use engine driven pumps, which, in time, can fail. Many classic and vintage airplanes have a venturi tube, which creates a vacuum as the slipstream flows through it - and which is prone to ice buildup. I have known applicants who assumed their classic airplanes had vacuum pumps, while in fact the venturi whistled away driving the gyros.
Modern instrument flight training becomes so procedure oriented that it's easy to minimize the systems aspect of your new IFR skills. The FAA wants pilots to understand aircraft systems. Some attitude indicators are electric and have a separate switch to energize them. Leaving the switch OFF while climbing into a low overcast is almost certain to lead to news reporters inspecting the accident site. To reduce the chance that this might happen, examiners ask questions such as, "Attitude indicators have what limitations?"
The PTS's Area of Operation II governs preflight procedures. Task B's first objective involves your knowledge of the elements related to applicable aircraft flight instrument systems and their operating characteristics. Added to the "Sacred Six" instruments are the magnetic compass, and a horizontal situation indicator (HSI) in place of a conventional directional gyro. If your aircraft has an HSI, be able to explain it as part of the aircraft's systems. But remember, the pitot-static system is always a vital background concern.
In its introduction, the PTS charges your examiner to make sure you demonstrate competency in either the primary-and-supporting, or the control-and-performance method of instrument flying. Both methods involve the same instruments, and both use the same responses for attitude control. They differ in their reliance on the attitude indicator, thus on the use and interpretation of other instruments. The Instrument Flying Handbook (AC 61-27) discusses these in Chapter 5. Flight instructors and examiners have their personal preferences, but examiners must test to the standards, not to their preference.
Your examiner might assume that you use the primary-and-supporting method, and during a maneuver ask which instruments are primary and which are supporting. If you learned the control-and-performance method, the question may seem odd. Should you be unable to answer, however, your examiner must test you on the control-and-performance concept.
The control-and-performance concept presents the question of attitude control. Airplane performance depends upon how a pilot controls the relationship of attitude to thrust. The first group of instruments (altimeter, heading indicator, vertical speed indicator, turn coordinator, and airspeed indicator) present existing performance information. The second group of instruments (attitude indicator, tachometer, or tachometer and manifold pressure gauge) show information you need to select the correct power and attitude to maintain or change airplane performance. The primary-and-supporting method addresses aircraft control from the standpoint of maneuvering about each axis. Attitude control involves pitch, bank, power, and trim control. thus, instruments are grouped as they relate to control function and airplane performance.
The examiner will watch your cross-check habits to evaluate the quality and quantity of information you get from the flight instruments and how you correlate that information to the task at hand. A too-rapid cross-check limits both the quality and quantity of information you receive. Too slow a cross-check leads to fixation or emphasis. A good cross-check speed allows you to observe, interpret, and respond, and it gives you the most timely ability to detect instrument failure.
B. TASK:
AIRCRAFT FLIGHT INSTRUMENTS AND NAVIGATION EQUIPMENT (IA, IH, A)
REFERENCES: FAR Parts 61, 91; AC 61-27
Objective. To determine that the applicant:
1. Exhibits adequate knowledge of the elements related to applicable aircraft flight instrument system(s) and their operating characteristics to include-
a. pitot-static.
b. altimeter.
c. airspeed indicator.
d. vertical speed indicator.
e. attitude indicator.
f. horizontal situation indicator.
g. magnetic compass.
h. turn-and-slip indicator/turn coordinator.
i. heading indicator.
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