ACS Task IR.II.A — Aircraft Systems Related to IFR Operations

What is ACS Task IR.II.A and how does the DPE test it?

ACS Task IR.II.A is the preflight oral knowledge evaluation covering the aircraft systems that enable IFR operations. Under the Instrument Rating ACS (FAA-S-ACS-8C) , you must explain how the pitot-static, vacuum, electrical, and avionics systems work — and critically, what happens when any of them fails.

The DPE frames this task around your specific aircraft, not a generic textbook diagram. Expect questions like: "Your pitot heat fails on departure and you climb into icing conditions — which instruments are at risk and what are your clues?" or "Walk me through what you'd see on your instruments if the static port iced over at 8,000 feet." Candidates who memorize system names without connecting them to failure-mode behavior consistently struggle here.

This task is the system layer beneath

, which covers the instruments those systems power. The DPE may blur the boundary between the two tasks, so preparation for IR.II.A should include IR.II.B and vice versa.

What does 14 CFR 91.205(d) require for IFR flight?

14 CFR 91.205(d) lists the minimum instruments and equipment required for IFR operations in powered civil aircraft. The regulation builds on the VFR day and night lists from 91.205(b) and (c), then adds the IFR-specific items:

• Two-way radio communication and navigation equipment suitable for the route to be flown
• Gyroscopic rate-of-turn indicator (or an approved third attitude instrument system for certain aircraft)
• Slip-skid indicator
• Sensitive altimeter adjustable for barometric pressure
• Clock displaying hours, minutes, and seconds with a sweep-second pointer or digital presentation
• Generator or alternator of adequate capacity
• Gyroscopic pitch and bank indicator (attitude indicator)
• Gyroscopic direction indicator (heading indicator or equivalent)

The DPE will ask you to recite this list from memory and, more importantly, to explain what each item does and why it is required specifically for IFR. The generator/alternator requirement exists because IFR flight depends entirely on electrically powered avionics — without charging capacity, battery depletion is a predictable emergency.

How do pitot-static blockages affect each instrument?

The pitot-static system supplies pressure inputs to the ASI, altimeter, and VSI. Understanding which instrument uses which port — and what happens when each port fails — is the most testable topic in IR.II.A, per FAA-H-8083-15B Chapters 4 and 7 .

Why the ASI behaves differently for each blockage: The ASI measures the differential between pitot (ram) pressure and static pressure. Blocked pitot with drain open removes ram pressure entirely — differential collapses to zero. Blocked pitot with drain also closed traps ram pressure; as altitude increases, the trapped air expands against static, which rises with altitude — the ASI behaves like an altimeter and shows a dangerously false increase. Blocked static with pitot normal keeps the ram side working but freezes the reference; as the aircraft climbs, ambient static falls below the trapped value, so differential shrinks and the ASI under-reads.

Alternate static source: Required equipment under 14 CFR 91.205(d) , the alternate static source vents the system to cabin air. Cabin air is at slightly lower pressure than external static, causing the altimeter to over-read and the ASI to over-read. Always consult the POH for aircraft-specific correction values.

What do vacuum and pressure systems power and what fails when they fail?

Most light aircraft use a vacuum pump or pressure system to spin the gyroscopes inside the attitude indicator (AI) and heading indicator (HI), as described in FAA-H-8083-15B Chapter 5 . The turn coordinator is typically electrically powered and operates on a separate bus from the vacuum gyros.

Vacuum failure indications: The suction gauge (required under 91.205(d)) is the primary warning. Most aircraft display a suction range of approximately 4.5–5.5 in. Hg for proper gyro operation. A failing vacuum pump may produce intermittent suction — a slowly processing AI or a HI that drifts faster than normal are early warning signs before the suction gauge pegs low.

Gyro spin-down time matters in IMC. A vacuum AI does not fail instantaneously — it may appear to function for several minutes after vacuum loss while the gyro decelerates. During this window the AI can give subtly false bank indications without any obvious flag. The discipline is to cross-check the AI against the turn coordinator and inclinometer before trusting either instrument alone.

Electric-gyro backup systems: Some aircraft replace the vacuum AI with a standby electric AI or an AHRS-based primary flight display. Know your specific aircraft's configuration — the DPE will ask which instruments are vacuum-driven and which are electric in the airplane you bring to the checkride.

What does the electrical system power and what happens when it fails?

Every electronic avionics component in a modern IFR aircraft — radios, transponder, GPS, autopilot, EFIS displays, electric AI, pitot heat — runs on the aircraft's electrical bus. The generator or alternator charges the battery and supplies load capacity in flight. The battery provides power for a limited time during alternator failure.

• Alternator failure: the master bus begins draining the battery immediately. Load shedding — turning off non-essential avionics — extends battery life. Identify and brief the order of equipment to shut down before flight.
• Battery-only endurance: most light aircraft batteries provide 20–45 minutes of power at reduced avionics load. Time to nearest airport matters; this is not an open-ended situation.
• Dual bus architecture: many IFR aircraft split avionics across a main bus and an avionics bus (controlled by the avionics master). Know which bus powers which equipment — a tripped avionics master can appear as a total avionics failure.
• Pitot heat: electrically powered, and a common single-point failure in icing conditions. Loss of pitot heat in IMC can produce pitot blockage within minutes in visible moisture at temperatures near freezing.
• Autopilot: computers the flight director and control servos from the same avionics bus. A degraded power supply can cause erratic autopilot behavior before a hard failure — one reason autopilot use in IMC requires active monitoring, not delegation.

What primary and secondary navigation systems does IR.II.A cover?

AIM Chapter 1-1 describes the FAA navigation infrastructure. For IR.II.A, the DPE expects you to identify the primary navigation system for your checkride, know its IFR regulatory requirements, and describe how the secondary system provides backup.

GPS/WAAS and RAIM: WAAS (Wide Area Augmentation System) uses a network of ground reference stations and a geostationary satellite overlay to correct GPS errors and provide integrity monitoring. WAAS-enabled receivers can achieve the accuracy needed for LPV approaches — which can have decision altitudes as low as 200 feet HAT, comparable to ILS Cat I. Without WAAS, you are limited to LNAV minimums. Confirm RAIM availability at your ETA during preflight; a predicted RAIM outage requires an alternate approach procedure.

Autopilot awareness: The autopilot is fed by the same sensors that drive the PFD. A failed AI, a degraded GPS signal, or an ADC error can cause the autopilot to track false data silently. The DPE expects you to know that the autopilot is a tool, not a backup system — its failure mode is the same as the primary sensor it is using.

What are the risk management elements for IR.II.A?

The ACS requires you to recognize and mitigate system failure risks before and during IFR flight. The DPE is listening for whether you reason about failure chains, not just individual components.

• Preflight system check — verify pitot heat operational, suction in the green, avionics bus functional, and alternator output normal before every IFR departure
• Pitot heat management — in icing conditions, pitot heat ON before entering visible moisture; a blocked pitot in IMC produces misleading ASI indications within minutes
• Vacuum failure planning — brief partial-panel procedures before IMC departure; know which instruments remain reliable and practice flying partial panel before you need it
• Electrical load management — know your battery endurance and the priority order for load shedding; many pilots have never rehearsed an alternator failure scenario
• Single-point navigation failure — if GPS is the only IFR navigation source and RAIM fails enroute, you may be unable to continue; file with alternates and carry paper charts for the route
• Autopilot coupling in degraded conditions — coupled approaches during sensor degradation can produce go-arounds or missed approaches if the pilot is not monitoring actively

What does the DPE look for when evaluating IR.II.A?

The DPE's standard for IR.II.A is failure-mode reasoning, not a memorized parts list. Per the Instrument Rating ACS (FAA-S-ACS-8C) , you must be able to connect a system failure to its cockpit presentation and your operational response.

1. 1
   State which instruments are affected by each pitot-static failure and describe the direction of error — not just 'the altimeter is wrong' but 'the altimeter freezes at the altitude where the static port blocked'
2. 2
   Identify whether each gyroscopic instrument in your aircraft is vacuum or electrically powered — the DPE will probe your specific airplane, not a generic example
3. 3
   Explain the alternator's role in IFR flight and describe your load-shedding priorities if the alternator fails in IMC
4. 4
   Describe the WAAS architecture at a conceptual level — ground stations, geostationary overlay, integrity signal — and distinguish WAAS from basic GPS RAIM
5. 5
   Demonstrate knowledge of the alternate static source: when to use it, which instruments it restores, and why the readings differ slightly from primary static
6. 6
   Articulate the autopilot's dependency on underlying sensors and state that autopilot failure is a workload event, not an emergency requiring immediate landing

What are the most common errors on IR.II.A?

• Mixing up pitot and static blockage effects on the ASI — the direction of error (over-read vs. under-read) depends on which scenario applies; know the table cold
• Stating that the altimeter uses pitot pressure — it does not; the altimeter and VSI connect only to the static system and are completely unaffected by pitot tube blockage
• Not knowing which instruments are vacuum vs. electric in your specific aircraft — this is aircraft-specific knowledge, not generic; research your POH before the checkride
• Treating autopilot failure as a single-system event — the DPE expects you to identify that autopilot performance depends on AI, GPS, and ADC health simultaneously
• Omitting the logging requirement for the VOR check under 14 CFR 91.171 — date, place, bearing error, and signature are all mandatory; forgetting any one element is a deficiency
• Confusing WAAS and RAIM — RAIM is an onboard receiver function using satellite geometry math; WAAS is an FAA ground network that augments GPS accuracy and provides its own integrity signal