Commercial pilot · ACS CA.VI.B · lesson pack

Navigation Systems and Radar Services

Why this one matters

Modern navigation is precise until it isn't: a wrong waypoint, a mode the pilot didn't notice, an expired database or a GPS outage. Commercial pilots in technically advanced airplanes are most at risk when they trust the box without cross-checking. Knowing each system's limits and having a backup plan is the safety margin.

1 · Know these cold

•Identify a VOR before using it, by its Morse code or its recorded voice identification (the name followed by "VOR"). No identifier, or the T-E-S-T code during maintenance, means don't use it.(AIM, Chapter 1, Air Navigation (navigation aids and GPS))
•VOR station passage: the TO/FROM flag flips. GPS: the waypoint sequences. An NDB: the needle swings from ahead to behind.(AIM, Chapter 1, Air Navigation (navigation aids and GPS))
•GPS integrity: RAIM is the receiver cross-checking redundant satellite signals for consistency (5 satellites, or 4 with baro-aiding, to detect a fault; 6, or 5 with baro-aiding, to exclude it; 4 for a 3D fix). WAAS is a separate augmentation with its own corrections and integrity. An integrity alert means the position may be wrong: cross-check another source.(AIM, Chapter 1, Air Navigation (navigation aids and GPS))
•Know what the automation is doing: verify each mode on the annunciator, and be ready to fly the airplane by hand when it does something unexpected.(Advanced Avionics Handbook (FAA-H-8083-6))
•Transponder (91.215): Class A, B and C; within 30 nm of a Class B primary airport, surface to 10,000 MSL; above a Class B or C ceiling within its lateral limits up to 10,000 MSL; and at and above 10,000 MSL everywhere in the 48 states and DC, except at or below 2,500 AGL. ADS-B Out (91.225) is similar but not identical: the same, except that above 10,000 MSL it applies only in Class E, and it adds Class E over the Gulf of Mexico at and above 3,000 MSL within 12 nm of the coast.(14 CFR 91.215(b), 91.225(d))
•Radar services (flight following) give traffic advisories and safety alerts workload permitting; you still see and avoid.(AIM, Chapter 4, Air Traffic Control (radar services for VFR aircraft))
•Commercial standard: intercept and track a course, recognize station passage and signal loss, altitude ±100 feet, heading ±10°.(Commercial Pilot ACS (FAA-S-ACS-7B), CA.VI.B.S3-S7)

2 · The lesson

Ground-based navigation

VOR: tune, identify by Morse code, set the course, read TO/FROM. The needle shows where the course is relative to you, regardless of heading.(AIM, Chapter 1, Air Navigation (navigation aids and GPS))
For IFR, the VOR must have been checked within 30 days (VOT or ground checkpoint ±4°, airborne checkpoint ±6°, dual VORs within 4° of each other), logged with date, place, error and signature.(14 CFR 91.171)
VOR signals are line of sight: low altitude, terrain and distance reduce reception. Each VOR has a standard service volume by class (terminal, low and high altitude, with the newer VL and VH tiers) defined in AIM 1-1-8; the Chart Supplement notes restrictions.(AIM 1-1-8)
DME gives slant range, not ground distance: it over-reads when you're close and high (overhead at 6,000 feet AGL it shows about 1 nm). It's usually paired with the VOR frequency.(AIM, Chapter 1, Air Navigation (navigation aids and GPS))
ADF and NDB: the needle shows the relative bearing to the station; magnetic heading plus relative bearing gives the magnetic bearing to it. Identify by Morse code; expect errors from thunderstorms, terrain and night effect.(AIM, Chapter 1, Air Navigation (navigation aids and GPS))

Satellite navigation

GPS needs at least 4 satellites for a 3D fix and more for integrity: RAIM needs 5 (or 4 with baro-aiding) to detect a fault and 6 (or 5) to exclude it. WAAS adds its own corrections and integrity. Heed integrity alerts and NOTAMed outages.(AIM, Chapter 1, Air Navigation (navigation aids and GPS))
Keep the database current; for VFR use, check waypoints against the chart, and remember a GPS can be spoofed or jammed near some areas (check GPS NOTAMs).(AIM, Chapter 1, Air Navigation (navigation aids and GPS))
An EFB with GPS position is a powerful situational-awareness tool; know its update cycle and keep a backup (battery, paper or second device).(AC 91-78A, Use of Electronic Flight Bags)

Radar services and surveillance

Basic radar service for VFR: safety alerts, traffic advisories, limited vectoring on request, and sequencing where established. Request it with your call sign, type, position, altitude and destination; it's workload permitting. TRSA service adds sequencing and separation for participating VFR; Class C service, IFR/VFR separation and VFR arrival sequencing; Class B, separation and sequencing. Lost or in trouble, ATC can give radar assistance and vectors.(AIM, Chapter 4, Air Traffic Control (radar services for VFR aircraft))
The requirement is a Mode 3/A (4096-code) or Mode S transponder plus altitude reporting (Mode C, in 100-foot increments); Mode S alone doesn't replace altitude reporting. It must have been tested within the preceding 24 calendar months (91.413). ADS-B Out broadcasts GPS position: Class A requires 1090ES; below 18,000 MSL either 1090ES or 978 MHz UAT meets 91.227. Aircraft certificated without an engine-driven electrical system (and gliders and balloons) have exceptions.(14 CFR 91.215, 91.225, 91.227, 91.413)
ADS-B In shows traffic and weather (FIS-B, TIS-B), but not every aircraft appears, and the weather isn't real time.(AIM, Chapter 7, Safety of Flight (in-flight weather and FIS-B))

Automation and its failures

Brief the automation before using it: what mode, what it will capture, what it will do next. Verify every change on the annunciator.(Advanced Avionics Handbook (FAA-H-8083-6))
Know each system's limits: VOR line of sight, GPS integrity, database currency, EFB battery and update age.(Advanced Avionics Handbook (FAA-H-8083-6))
If a signal is lost or flags: fly the airplane, revert to another source (VOR, GPS, pilotage), tell ATC if you're on a clearance or flight following.(AIM, Chapter 1, Air Navigation (navigation aids and GPS))
Heads-down programming takes your eyes out of the sky: program on the ground or in level flight, in short steps, with frequent scans outside.(AC 90-48, Pilots' Role in Collision Avoidance)

Want it all on one page, with scenarios? Read the full lesson.

On the flight

ACS standards

Use an airborne electronic navigation system; determine position; intercept and track a course, radial or bearing; recognize station or waypoint passage.(Commercial Pilot ACS (FAA-S-ACS-7B), CA.VI.B.S1-S4)
Recognize signal loss or interference and act; communicate properly with radar services; altitude ±100 feet, heading ±10°.(Commercial Pilot ACS (FAA-S-ACS-7B), CA.VI.B.S5-S7)

What your instructor is watching for

Your instructor watches for stations identified, intercepts flown cleanly, station passage recognized, automation modes verified, a backup ready when a signal is lost, and correct flight-following phraseology, all within commercial tolerances.

3 · Say it out loud

Answer each one out loud, then check yourself and mark how it went.

Remember Recall it
“How do you know a VOR is usable?”
Understand Explain why
“Why cross-check GPS against another source?”
Apply Use it on a flight
“Cruising on autopilot with GPS, you get an integrity alert. What do you do?”

4 · Common mistakes

Not identifying the station.
You could be on the wrong station or one under maintenance.
Instead: Listen to the Morse code every time.
Trusting the autopilot without checking the mode.
Mode confusion leads to deviations.
Instead: Verify the annunciator after every change.
Treating flight following as a guarantee.
Advisories depend on workload and radar coverage.
Instead: Keep scanning and see and avoid.

5 · Quick check

Answer, then reveal. Flag anything you want to see again.

1. What are the IFR VOR check tolerances for a VOT and an airborne checkpoint?
2. What does RAIM do?
3. Where is ADS-B Out required?
4. How do you recognize VOR station passage?
Scenario5. Your EFB battery dies on a cross-country. What should you have?

Trained it? Now prove it: practice navigation systems and radar services out loud with our examiner.

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Connect it

One situation that runs across this lesson and its neighbors in the same ACS area. Decide first, then check.

Connect it · Navigation

A night cross-country. Headwinds are stronger than forecast and your groundspeed is down 25 knots.

How do your checkpoint times feed a diversion decision? Decide, then see the worked answer

Compare the actual time at each checkpoint with the plan. A 25-knot shortfall stretches every leg, so recompute the fuel to the destination at the actual groundspeed. Keep the reserve separate: 91.151(a)(2) requires enough fuel at night to reach the destination and then fly 45 more minutes at normal cruising speed, so add 45 minutes at normal cruise on top of the destination fuel. That's the legal floor; a personal minimum can raise it but never lower it. If you'd land with less than your minimum, divert now, while you still know exactly where you are. At night, choose the alternate with the Chart Supplement: runway lighting and how it's activated, runway length and surface, and whether fuel is actually available at that hour. Note the time and heading as you turn, estimate heading, time and fuel to the new airport, then refine. Leaving it late, at night, is how being short of fuel turns into being lost.

14 CFR 91.151; PHAK (FAA-H-8083-25C), Navigation; Chart Supplement

Brings together: Pilotage and Dead Reckoning · Diversion

Teaching this? The instructor lesson plan →

Study aid, not FAA guidance. Sources: Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C); Advanced Avionics Handbook (FAA-H-8083-6); Aeronautical Information Manual (AIM); 14 CFR Part 91; AC 90-48 and AC 91-78; Commercial Pilot ACS (FAA-S-ACS-7B); 14 CFR 91.171. Checked against current sources 2026-11-13.