Cessna 172 (Analog Six-Pack) — Instrument Checkride Guide

What IFR-relevant systems does the analog Cessna 172 have?

The analog C172 uses a six-pack instrument layout that the FAA Instrument Flying Handbook (FAA-H-8083-15B) describes as two groups: the pitot-static instruments on the left column (airspeed indicator, altimeter, vertical speed indicator) and the gyroscopic instruments on the right (attitude indicator, heading indicator, turn coordinator). Understanding which group each instrument belongs to — and what powers it — is foundational to every partial-panel question a DPE will ask.

The pitot-static instruments derive their readings entirely from ram air pressure at the pitot tube and ambient pressure at the static port. They require no electrical or vacuum power. An alternate static source, selectable from the cockpit, routes cabin air to these instruments when the primary static port is blocked.

The gyroscopic instruments split into two power sources. The AI and HI receive suction from the engine-driven vacuum pump. The turn coordinator has its own electrical motor. This means pitot-static failures and vacuum failures produce completely different instrument outage patterns — a distinction the DPE will expect you to articulate precisely.

• Airspeed indicator, altimeter, VSI — pitot-static; no electrical or vacuum dependency
• Attitude indicator (AI) — vacuum-driven gyro; fails silently when vacuum is lost
• Heading indicator (HI) — vacuum-driven gyro; fails with the AI on vacuum failure
• Turn coordinator — electrically driven; survives vacuum failure
• Magnetic compass — self-contained; survives both vacuum and electrical failure
• Alternate static source — bypasses blocked primary static port using cabin air

What avionics suites are common in analog C172 trainers?

The navigation radio installed in any specific aircraft varies by year and owner, so confirm your aircraft's actual equipment before the checkride. That said, the analog C172 training fleet clusters around a handful of common configurations.

The Bendix/King KX 155 and KX 165 nav/com units are the most prevalent navigation radios in this fleet. They provide VHF navigation (VOR and localizer) and ILS glideslope reception. For GPS, many aircraft have been upgraded with a Garmin GNS 430 or GNS 530; some older aircraft carry a Bendix/King KLN 89 or KLN 94 GPS navigator. GPS navigators that are certified for IFR enroute and approach use will be noted on the aircraft's equipment list and AFM supplement.

If a KAP 140 two-axis autopilot is installed, you are expected to understand its modes (heading, altitude, approach), its annunciations, and how to disengage it — including in a runaway trim scenario. The DPE will not let unfamiliarity with your own autopilot pass without comment.

How does the vacuum system work, and what fails when it does?

The engine-driven vacuum pump draws air through the AI and HI gyros, spinning them to operating speed via the resulting airflow. The FAA Instrument Flying Handbook, Chapter 5 describes the suction gauge — typically marked with a green arc — as the primary tool for monitoring vacuum system health. Normal operating suction is specified in your aircraft's POH; a reading below the green arc indicates degraded or absent vacuum.

Vacuum failure is insidious because the AI and HI do not fail immediately — they coast on stored gyroscopic energy for several minutes before tumbling or showing erroneous indications. A pilot relying on an AI that is slowly precessing toward a bank may chase the instrument deeper into an upset before noticing the suction gauge has dropped to zero.

When the vacuum pump fails:

• Attitude indicator — loses vacuum; gyro coasts, then precesses or tumbles; indications become unreliable
• Heading indicator — loses vacuum; gyro precesses; heading indications drift and eventually become meaningless
• Turn coordinator — unaffected; electrically driven; becomes primary roll reference for partial panel
• Magnetic compass — unaffected; becomes primary heading reference for partial panel
• Airspeed, altimeter, VSI — unaffected; pitot-static instruments with no vacuum dependency

This failure pattern is the direct subject of ACS Task IR.VII.C (partial panel instrument approach). The DPE will cover this in the oral and demonstrate it on the flight portion. See the ACS IR.VII.C partial panel guide for scan substitution technique, magnetic compass error corrections, and ACS tolerances.

What are the common DPE oral questions for the analog Cessna 172?

DPEs examining analog C172 applicants focus heavily on system knowledge — specifically which instruments fail under which failure modes — and on the regulatory requirements for IFR equipment. The questions below represent the range of topics covered under ACS task area II (aircraft systems) and are commonly used to probe partial-panel readiness.

• "Walk me through what fails when your vacuum pump fails in IMC."
• "Your AI is showing a 30-degree left bank but your turn coordinator shows wings level. Which do you trust, and why?"
• "What is the first indication you would see of vacuum system failure in the cockpit?"
• "How does your alternate static source affect your altimeter and airspeed readings?"
• "Your pitot heat is inoperative and forecast icing conditions exist along the route. What does 14 CFR 91.205 say about this?"
• "Walk me through the electrical bus — what avionics lose power in a complete alternator failure?"
• "Your KAP 140 is in approach mode and fails to track the glideslope. How do you disengage and continue?"
• "Is your GPS database current, and what happens to your approach authorization if it expires mid-trip?"
• "When was your VOR last checked, and where is that logged?"
• "Describe the pitot-static system from the pitot tube to the instrument — what happens if the drain hole is blocked by ice?"

Prepare specific, system-level answers for each. For failure-mode questions, trace the failure path explicitly: which component fails, what the first observable indication is, which instruments are affected, and what you do next.

What are the VOR check and database currency requirements?

For VOR navigation, 14 CFR 91.171 requires that any VOR used for IFR operations be checked within the preceding 30 days. The log entry must include the date, place, bearing error, and the pilot's signature. Maximum allowable error is ±4 degrees at a VOR test facility (VOT) or certified ground checkpoint, and ±6 degrees at an airborne checkpoint.

If a GPS navigator is installed and approved for IFR approaches, the navigation database must be current for approach use. Most IFR GPS units update on the FAA's 28-day AIRAC cycle. An expired database removes GPS approach authorization; enroute GPS use may continue only if the pilot can verify procedures have not changed, per applicable AFM supplement guidance.

The altimeter and static system inspection under 14 CFR 91.411 and the transponder inspection under 14 CFR 91.413 must both be within 24 calendar months. Verify these entries in the aircraft logbooks before your checkride — the DPE will inspect them and may ask you to locate each entry directly.

Practice Questions

Frequently Asked Questions

Q: Which instruments in the analog C172 are driven by the vacuum system?

The attitude indicator (AI) and the directional gyro/heading indicator (HI) are both vacuum-driven in the analog Cessna 172. The turn coordinator is electrically driven, and the airspeed indicator, altimeter, and VSI are pitot-static instruments. A vacuum failure takes out the AI and HI but leaves the remaining four instruments operational.

Q: What causes the vacuum system to fail in a Cessna 172?

The most common cause is failure of the engine-driven vacuum pump itself, which can fail without warning. Other causes include a broken vacuum line, a clogged filter, or a failed vacuum regulator. The suction gauge on the instrument panel is the primary indicator — a drop below the normal green arc indicates reduced or absent vacuum.

Q: How do you recognize vacuum system failure in flight?

Initial recognition usually comes from the suction gauge dropping out of the green arc, followed by the AI and HI gradually tumbling or showing erroneous indications. The AI may appear to show a slow, uncommanded bank. Cross-checking with the turn coordinator (electric) and magnetic compass provides the attitude and heading reference needed to continue flight.

Q: What is the alternate static source used for in the analog C172?

The alternate static source provides cabin air to the pitot-static instruments when the primary external static port is blocked by ice or debris. Because cabin pressure is slightly lower than ambient at cruise, activating alternate static may cause a slight increase in altimeter reading and airspeed indication. Consult your aircraft's POH for specific correction values.

Q: Is a VOR check required in the analog Cessna 172?

Yes. Under 14 CFR 91.171 , any VOR equipment used for IFR operations must be checked within the preceding 30 days. The check must be logged with the date, place, bearing error, and the pilot's signature. Maximum allowable error is ±4 degrees at a ground checkpoint and ±6 degrees at an airborne checkpoint.

Q: What avionics suites are common in analog C172 trainers?

The most common navigation radios are the Bendix/King KX 155 or KX 165 nav/com units. GPS navigators vary widely — the Garmin GNS 430 and GNS 530 are common upgrades. Some aircraft have a KLN 89 or KLN 94 GPS. The KAP 140 is a widely installed two-axis autopilot in this fleet.

Q: What IFR equipment does 14 CFR 91.205(d) require for the analog C172?

For IFR flight under 14 CFR 91.205(d) , the aircraft must have (among other items): a gyroscopic rate-of-turn indicator, a slip/skid indicator, a sensitive altimeter, a clock, a generator or alternator, and a gyroscopic pitch-and-bank indicator. In practice for the analog C172, this means the full six-pack plus a functioning vacuum system to power the AI and HI.

Q: What autopilot limitations apply during an instrument checkride?

If a KAP 140 or similar autopilot is installed, the DPE may allow its use on certain segments but will require at least one approach hand-flown to ACS tolerances. The DPE evaluates whether you can correctly engage, monitor, and disengage the autopilot, and whether you understand its failure modes — including runaway trim or failure to capture a glideslope.

Sources

• 14 CFR 91.205 — Instrument and Equipment Requirements
• 14 CFR 91.171 — VOR Equipment Check for IFR Operations
• 14 CFR 91.411 — Altimeter System and Altitude Reporting Equipment Tests
• 14 CFR 91.413 — ATC Transponder Tests and Inspections
• 14 CFR 91.409 — Annual Inspection Requirements
• FAA Instrument Flying Handbook (FAA-H-8083-15B)
• FAA Instrument Rating ACS (FAA-S-ACS-8C)
• FAA Instrument Procedures Handbook (FAA-H-8083-16B)

This article was researched from FAA primary sources (ACS, FAR/AIM, Instrument Flying Handbook) and citing current 14 CFR Part 91 — drafted by MockDPE. V-speeds and POH-specific values are not cited here because they vary by model year (172N, 172P, 172Q, 172R, 172S, 172SP) — always consult your specific aircraft's POH and AFM supplements. Last updated: May 2026. If you spot an inaccuracy, email corrections@mockdpe.org.