Reference Library

These reference pages consolidate the key knowledge areas for instrument flying. Use them for study, review, or quick lookup during lesson preparation.

Precision Approaches

A precision approach provides both lateral (course) and vertical (glide path) guidance to a Decision Altitude (DA). The pilot descends continuously — there is no level-off. At DA, you either have the required visual reference and land, or execute the missed approach.

Type	Description
ILS	Instrument Landing System — localizer (lateral) + glideslope (vertical). Most common precision approach. Localizer: 2.5° full-scale. Glideslope: typically 3°, 0.7° full-scale.
PAR	Precision Approach Radar — controller provides course and glidepath guidance via radio. Military airports primarily. "No gyro" approaches also available.
GLS	GBAS Landing System — ground-based augmentation of GPS. Provides ILS-equivalent guidance using corrected GPS signals. Category I, II, and III capable.

Non-Precision Approaches

A non-precision approach provides lateral guidance only — no electronic glide path. The pilot descends to a Minimum Descent Altitude (MDA) and levels off, continuing to the missed approach point while looking for the runway environment.

Type	Description
VOR	Course guidance from a VOR station. CDI sensitivity: 10° full-scale.
NDB	Course guidance from a non-directional beacon via ADF.
LOC	Localizer only — same as ILS lateral guidance without glideslope. CDI: 2.5° full-scale.
ASR	Airport Surveillance Radar — controller provides azimuth guidance only. No glidepath.
LDA	Localizer-type Directional Aid — same signal as a localizer but not aligned with the runway (offset more than 3°). May or may not have glideslope.
SDF	Simplified Directional Facility — similar to localizer but wider course (6° or 12°) and less precise.
LP	Localizer Performance — WAAS lateral guidance only, no vertical. Tighter than LNAV.
LNAV	Lateral Navigation — GPS lateral guidance to MDA. 0.3 NM CDI sensitivity in terminal area.

Precision-Like Approaches (APV)

Approach with Vertical Guidance (APV) provides both lateral and vertical guidance but does not meet ICAO precision approach standards. These approaches use a Decision Altitude (DA) — not MDA — because of the continuous descent guidance.

Type	Description
Baro-VNAV	Vertical guidance computed from barometric altitude. Temperature-sensitive — not authorized below certain temperatures (noted on chart).
LNAV/VNAV	GPS lateral + barometric or WAAS vertical guidance. DA typically higher than LPV.
LPV	Localizer Performance with Vertical guidance — WAAS-based. Sensitivity comparable to ILS. Lowest minimums of any GPS approach type. DA can be as low as 200 ft AGL.
LDA with GS	LDA approach with a glideslope added. Flown to DA. Uncommon but exists at select airports.

LOC Approach Details

• LOC (front course): Standard localizer approach — fly toward the runway on the front course of the localizer. CDI works normally.
• LOC BC (back course): Approach using the back side of the localizer signal. CDI sensing is reversed unless the HSI or GPS compensates. Glideslope is NOT usable on back course.
• LDA: Localizer-type signal offset more than 3° from the runway centerline. Used when terrain or obstacles prevent runway alignment. CDI sensitivity same as localizer.
• SDF: Wider course than localizer (6° or 12° course width). Less precise, used where a localizer cannot be installed. May not be aligned with runway.

GPS Approach Types

Type	Guidance	Minimum Type	Equipment Required
LNAV	Lateral only	MDA	IFR-approved GPS
LNAV/VNAV	Lateral + vertical	DA	WAAS or Baro-VNAV capable GPS
LPV	Lateral + vertical (precision-like)	DA	WAAS GPS
LP	Lateral only (tighter than LNAV)	MDA	WAAS GPS

Approach Categories

Approach category is based on 1.3 V_SO (stall speed in landing configuration) at maximum certificated gross weight. If you fly faster, use the higher category's minimums.

Category	Speed Range (1.3 VSO)	Typical Aircraft
A	0–90 kts	Cessna 172, Piper Cherokee
B	91–120 kts	King Air, Bonanza, larger twins
C	121–140 kts	Business jets, regional turboprops
D	141–165 kts	Airliners, large jets
E	166+ kts	Military, special category

Circling Approaches

A circling approach is used when the final approach course is aligned more than 30° from the runway (15° for RNAV approaches), or when a straight-in landing is not feasible. The pilot descends to the circling MDA, establishes visual contact, and maneuvers to land on the appropriate runway.

Circling Approach Rules

• Obstacle clearance: 300 ft within the circling area
• Remain within the circling area at all times while below the circling MDA
• If visual reference is lost: Execute a climbing turn toward the landing runway to intercept the missed approach course
• Night circling: Exercise extreme caution — reduced visual cues. Some approaches prohibit circling at night for specific categories.

Circling Approach Radii

Category	Standard Radius	Expanded Radius (new criteria)
A	1.3 NM	1.7 NM
B	1.5 NM	2.3 NM
C	1.7 NM	3.1 NM
D	2.3 NM	4.0 NM
E	4.5 NM	5.3 NM

Straight-In Criteria

A straight-in approach requires:

• Course alignment: Final approach course within 30° of the runway centerline (15° for RNAV)
• Descent gradient: No more than 400 ft/NM from the FAF to the threshold

If either criterion is not met, only circling minimums are published. An approach titled "VOR Rwy 24" has straight-in minimums; "VOR-A" is circling only.

Departure Procedure Fundamentals

• 35 ft AGL: Assumed crossing height at the departure end of the runway
• 400 ft before turning: Climb to at least 400 ft AGL before making the initial turn (unless otherwise specified)
• 200 ft/NM: Standard climb gradient for obstacle clearance (approximately 3.3° climb angle)

ODP vs SID

Feature	ODP (Obstacle Departure Procedure)	SID (Standard Instrument Departure)
Purpose	Obstacle clearance	ATC traffic flow and workload reduction
Published as	Text (in TPP) or graphic	Always graphic (chart)
ATC clearance required?	No — pilot's discretion (Part 91)	Yes — must be assigned by ATC
Mandatory?	No for Part 91 (strongly recommended). Yes for 121/135.	Yes — when assigned
Can be declined?	N/A — voluntary	Yes — "No SIDs" in remarks or tell ATC
Climb gradient	Standard 200 ft/NM unless noted	May require non-standard gradients

DP Equipment Categories

Departure procedures may require specific equipment. Look for notes on the chart or in the text:

• Standard: No special equipment beyond standard IFR
• RNAV: Requires GPS or RNAV capability
• DME: Requires DME for distance-based restrictions
• Radar: Requires radar contact with ATC

Chart Symbols

Symbol	Meaning
T (black triangle)	Non-standard takeoff minimums exist — check the TPP for details
A (black triangle)	Non-standard alternate minimums exist
A-NA	Airport is NOT AUTHORIZED as an alternate

VCOA — Visual Climb Over Airport

A departure option that allows you to climb visually over the airport to a specified altitude before proceeding on course. Used when the standard climb gradient cannot be met or when obstacles prevent a straight-out departure.

• Requires visual conditions at the airport
• Climb over the airport to the published altitude
• Then proceed on course or as assigned by ATC
• Published in the IFR takeoff minimums and departure procedures section

Diverse Departure / DVA

A diverse departure means no ODP is published because the standard 200 ft/NM gradient provides adequate obstacle clearance on any heading. If an airport has no published ODP and no "T" symbol, a diverse departure is available — you can depart on any heading with the standard climb gradient.

A Diverse Vector Area (DVA) allows ATC to vector departing aircraft in any direction, provided they meet a specified climb gradient. Published when radar vectors are available and the area is evaluated for obstacle clearance.

STAR — Standard Terminal Arrival Route

• Definition: A published IFR arrival route from the en route structure to a fix in the terminal area, simplifying clearance delivery and reducing radio congestion.
• Transition routes: STARs begin with transition fixes that connect to the en route structure. Multiple transitions may feed into a common STAR route.
• RNAV requirement: Many modern STARs require RNAV capability (GPS). Check the chart notes.
• Declining a STAR: Write "No STARs" in the remarks section of your flight plan, or tell ATC. When assigned, compliance is mandatory.
• Speed/altitude restrictions: STARs often include "descend via" clearances with published speed and altitude restrictions at each waypoint.

IFR Takeoff Minimums

Operation	Standard Takeoff Minimums
Part 91	None required — Part 91 pilots may depart in zero-zero conditions (strongly discouraged). Must be able to comply with departure procedure climb gradients.
Part 121 / 135	1 SM visibility for 1 or 2 engine aircraft. 1/2 SM for 3+ engines. Non-standard minimums apply when published (check "T" symbol).

FPNM to FPM Conversion

Departure procedures publish climb gradients in feet per nautical mile (FPNM). To convert to a usable feet per minute (FPM) rate of climb:

Always use groundspeed (not airspeed) because the gradient is based on distance over the ground. Headwinds reduce your groundspeed, requiring a higher FPM to achieve the same FPNM gradient.

Gyroscopic Instruments

Gyroscopic instruments use the properties of rigidity in space and precession to sense aircraft attitude and rate of turn.

Attitude Indicator (AI)

• Principle: Rigidity in space — the gyro remains fixed while the aircraft moves around it
• Shows: Bank angle and pitch attitude relative to the horizon
• Power source: Vacuum/pressure pump (most GA aircraft) or electric
• Limitations: Tumbles at extreme attitudes (beyond ~60° bank or ~30° pitch in older models). Errors during acceleration (shows slight pitch up) and turns (precession drift)

Heading Indicator (HI)

• Principle: Rigidity in space — gyro maintains orientation while aircraft yaws
• Shows: Aircraft heading (no magnetic seeking — must be calibrated with magnetic compass)
• Calibration: Set to magnetic compass every 15 minutes in straight, unaccelerated flight
• Power source: Vacuum/pressure pump
• Limitation: Precesses (drifts) approximately 3° per 15 minutes due to mechanical friction and Earth rotation

Turn Indicators

• Principle: Precession — force applied to a spinning gyro is felt 90° in the direction of rotation
• Turn Coordinator (TC): Gyro canted 30°, senses both roll rate and yaw rate. Miniature airplane symbol. Standard rate turn = 3°/sec.
• Turn-and-Slip Indicator: Gyro aligned with lateral axis, senses yaw rate only. Vertical needle deflection.
• Inclinometer (ball): Not gyroscopic — a gravity-based slip/skid indicator. Step on the ball to coordinate.
• Power source: Electrically driven (provides redundancy if vacuum fails)

Pitot-Static Instruments

Three instruments connected to the pitot-static system: the altimeter, VSI, and airspeed indicator.

Altimeter

• Type: Aneroid barometer — measures static pressure against sealed aneroid wafers
• Kollsman window: Set the current altimeter setting (inches of mercury) to display correct indicated altitude
• Rule: "High to low or hot to cold, look out below" — flying from high pressure to low pressure without resetting causes the altimeter to read higher than actual altitude

Five Types of Altitude

Type	Definition
Indicated	Read directly from altimeter set to local altimeter setting
Pressure	Indicated altitude when set to 29.92" Hg (standard datum)
Density	Pressure altitude corrected for non-standard temperature
True	Actual height above mean sea level (MSL)
Absolute	Height above ground level (AGL) — measured by radar altimeter

Vertical Speed Indicator (VSI)

• Measures: Rate of altitude change in feet per minute (FPM)
• Lag: Standard VSI has a 6–9 second lag due to the calibrated leak in the instrument case
• IVSI: Instantaneous VSI uses accelerometer pumps to reduce lag to 1–2 seconds
• Use in IFR: Trend instrument — verify climbs, descents, and level flight. Not used to chase specific FPM targets.

Airspeed Indicator (ASI)

• Measures: Dynamic pressure (ram air pressure minus static pressure) via a diaphragm in the instrument case
• Pitot tube provides: Total pressure (ram + static)
• Static port provides: Ambient static pressure

Types of Airspeed

Abbreviation	Name	Definition
IAS	Indicated Airspeed	Read directly from the ASI
CAS	Calibrated Airspeed	IAS corrected for instrument and position error
EAS	Equivalent Airspeed	CAS corrected for compressibility (significant above 200 kts)
TAS	True Airspeed	EAS corrected for density altitude — actual speed through the air mass
Mach	Mach Number	TAS divided by the local speed of sound
GS	Groundspeed	TAS adjusted for wind — actual speed over the ground

ASI Markings

Marking	Color	Range
VSO to VFE	White arc	Full flap operating range (stall speed in landing config to max flap extended speed)
VS1 to VNO	Green arc	Normal operating range (clean stall speed to max structural cruising speed)
VNO to VNE	Yellow arc	Caution range — smooth air only
VNE	Red line	Never exceed speed

System Blockage Effects

Static Port Blocked

• Altimeter: Freezes — shows altitude at which blockage occurred
• VSI: Freezes at zero
• ASI: Reads inaccurately — works like an altimeter (reads high when climbing, low when descending) because ram pressure changes while static pressure is trapped

Pitot Tube Blocked (ram air only, drain open)

• ASI: Drops to zero as ram pressure equalizes
• Altimeter & VSI: Unaffected (static-only instruments)

Pitot Tube Blocked (ram air AND drain)

• ASI: Acts like an altimeter — pressure is trapped in the pitot line, so the ASI reads higher as altitude increases and lower as altitude decreases, regardless of actual airspeed

Alternate Static Source (cabin air)

• Cabin pressure is slightly lower than outside static due to aerodynamic effects
• Altimeter: Reads slightly high
• ASI: Reads slightly high
• VSI: Momentary climb indication, then normal

Magnetic Compass Errors — DV MONA

Error	Description
D — Deviation	Caused by magnetic interference from aircraft electronics, engine, and metals. Corrected with compass deviation card.
V — Variation	Difference between true north and magnetic north. Found on sectional chart isogonic lines.
M — Magnetic dip	Compass card tilts toward the nearer magnetic pole. Causes turning and acceleration errors in the northern hemisphere.
O — Oscillation	Compass card swings due to turbulence. Read the average.
N — Northerly turning error	On N/S headings: compass leads in S turns, lags in N turns. "UNOS" — Undershoot North, Overshoot South.
A — Acceleration error	On E/W headings: "ANDS" — Accelerate North, Decelerate South. The compass falsely indicates a turn toward north when accelerating.

Glass Cockpit Systems

AHRS — Attitude and Heading Reference System

Solid-state replacement for mechanical gyros. Uses accelerometers, rate sensors, and magnetometers to compute attitude and heading. No moving parts, no tumble limits, no vacuum system required.

ADC — Air Data Computer

Receives pitot and static pressure inputs and computes altitude, airspeed, vertical speed, and outside air temperature. Provides digital data to the PFD and other systems.

EFIS — Electronic Flight Instrument System

Integrates AHRS and ADC data onto glass displays. Typically includes a Primary Flight Display (PFD) for attitude, airspeed, altitude, heading, and VSI, plus a Multi-Function Display (MFD) for navigation, engine data, and weather.

Instrument Taxi Check

Before takeoff, verify instruments during taxi:

• Heading Indicator: Set to runway heading or compass. Should track turns during taxi.
• Attitude Indicator: Erect within 5 minutes. Shows slight bank in turns.
• Turn Coordinator: Deflects in direction of turn during taxi. Ball moves to outside of turn.
• Magnetic Compass: Agrees with known taxi heading.
• Altimeter: Within 75 feet of field elevation when set to current altimeter setting.

Minimum IFR Altitudes (14 CFR 91.177)

When operating under IFR in controlled airspace, the minimum altitude depends on terrain type:

Terrain Type	Minimum Clearance	Lateral Distance
Mountainous	2,000 ft above highest obstacle	Within 4 NM of course
Non-mountainous	1,000 ft above highest obstacle	Within 4 NM of course

These minimums apply when flying off published airways or routes that have their own published MEAs.

Altitude Definitions

Abbreviation	Name	Definition
DA / DH	Decision Altitude / Decision Height	The altitude (MSL) or height (AGL) on a precision or APV approach at which a missed approach must be initiated if the required visual reference is not established. You do not level off — it is a decision point during descent.
MDA / MDH	Minimum Descent Altitude / Height	The lowest altitude (MSL) or height (AGL) on a non-precision approach to which descent is authorized without visual contact. You level off at MDA and wait for visual reference or the MAP.
MEA	Minimum En Route Altitude	Lowest published altitude on an airway segment that guarantees: (1) obstacle clearance, (2) acceptable navigation signal coverage, and (3) ATC communications.
MOCA	Minimum Obstruction Clearance Altitude	Lowest published altitude that provides obstacle clearance on an airway segment. Navigation signal coverage guaranteed only within 22 NM of a VOR.
MRA	Minimum Reception Altitude	Lowest altitude required to receive adequate navigation signals to identify a fix on an airway.
MCA	Minimum Crossing Altitude	Lowest altitude at which a fix must be crossed when proceeding to a higher MEA route segment. Ensures obstacle clearance during the climb.
MAA	Maximum Authorized Altitude	Highest altitude on an airway at which adequate signal coverage is assured. Prevents receiving signals from wrong-frequency stations.
MTA	Minimum Turning Altitude	Published altitude required at certain waypoints to account for turn radius and obstacle clearance during RNAV turns.
MVA	Minimum Vectoring Altitude	Lowest altitude at which ATC can vector IFR aircraft. Not published on charts — used by controllers internally. Provides 1,000 ft (non-mountainous) or 2,000 ft (mountainous) obstacle clearance.
OROCA	Off-Route Obstruction Clearance Altitude	Published on en route charts. Provides 1,000 ft (non-mountainous) or 2,000 ft (mountainous) clearance in each lat/long grid. Does NOT guarantee navigation signal coverage.
MORA	Minimum Off-Route Altitude	Jeppesen equivalent of OROCA. Provides 1,000 ft clearance within 10 NM of route centerline (2,000 ft in mountainous areas).

MEA vs MOCA — The 22 NM Rule

The key difference: MOCA guarantees VOR navigation signal only within 22 NM of the VOR. Beyond 22 NM, you have obstacle clearance but may lose the navigation signal. MEA guarantees both obstacle clearance and navigation signal throughout the entire airway segment.

If ATC assigns you an altitude at the MOCA (between MOCA and MEA), you must accept it, but be aware of the navigation signal limitation. With GPS, signal coverage is not a factor.

Cruise Clearance vs "Maintain"

Clearance Type	Meaning
"Maintain 6,000"	Fly at 6,000 ft. You may not deviate from this altitude without ATC clearance.
"Cruise 6,000"	You may fly at any altitude from the MEA up to and including 6,000 ft. You may climb or descend at your discretion within that range. Once you report leaving an altitude, you may not return to it without ATC clearance. Also authorizes an approach at the destination.

IFR Cruising Altitudes (14 CFR 91.179)

When operating in controlled airspace under IFR on a magnetic course, use:

Magnetic Course	Below FL180	FL180–FL410
0°–179° (Eastbound)	Odd thousands (3,000, 5,000, 7,000...)	Odd flight levels (FL190, FL210, FL230...)
180°–359° (Westbound)	Even thousands (4,000, 6,000, 8,000...)	Even flight levels (FL200, FL220, FL240...)

Note: ATC can assign any altitude regardless of direction. These rules apply when you select your own altitude on an IFR flight plan.

Block Altitude

A block altitude clearance allows you to fly within a range of altitudes without ATC coordination for each altitude change. Example: "Maintain block altitude 5,000 to 7,000." You may operate at any altitude within the block but must report leaving one altitude for another within the block.

When Is an Instrument Rating Required?

Under 14 CFR 61.3(e), you must hold an instrument rating to act as PIC under IFR. Specifically, an instrument rating is required in these four situations:

1. Flight under IFR — operating on an IFR flight plan in controlled airspace (14 CFR 61.3(e))
2. Flight in weather below VFR minimums — when conditions are less than those prescribed for VFR flight (14 CFR 91.157)
3. Class A airspace — all operations above FL180 require an IFR clearance (14 CFR 91.135)
4. Special VFR at night — SVFR at night requires an instrument rating and instrument-equipped aircraft (14 CFR 91.157(b)(4))

Experience Requirements (14 CFR 61.65)

To be eligible for an instrument rating — airplane, you must have:

• 50 hours cross-country PIC — at least 50 hours of cross-country flight time as pilot in command (10 of which must be in airplanes)
• 40 hours actual or simulated instrument time — including at least 15 hours of instrument flight training from an authorized instructor (CFII)
• 3 hours instrument training within 2 calendar months preceding the practical test
• 250 NM cross-country flight under IFR — along airways or ATC-directed routing, with an instrument approach at each of 3 different airports

The 6 HITS Currency Rule (14 CFR 61.57(c))

To act as PIC under IFR or in weather conditions less than VFR minimums, within the preceding 6 calendar months you must have performed and logged:

Requirement	Quantity	Details
H — Holding procedures	1	Holding pattern entry and tracking
I — Intercepting	—	Intercepting courses (through holding and approaches)
T — Tracking	—	Tracking courses (through holding and approaches)
S — Six approaches	6	6 instrument approaches (can be in actual IMC, simulated, or ATD)

Currency Timeline

Period	Status	Action Required
Months 0–6	Current	Fly IFR as PIC. Perform 6 HITS to reset the clock.
Months 6–12	Grace period	Cannot fly IFR as PIC. Complete 6 HITS with a safety pilot or in an ATD to regain currency. No IPC required.
Beyond 12 months	Lapsed — IPC required	Must pass an Instrument Proficiency Check (IPC) with a CFII, examiner, or check airman before exercising IFR privileges.

Logging Instrument Time

• Actual instrument time — log only the time you are flying solely by reference to instruments in actual IMC.
• Simulated instrument time — log time under the hood (view-limiting device) with a safety pilot aboard. The safety pilot must hold at least a private pilot certificate with category and class ratings for the aircraft.
• Log each approach — record the location (airport identifier) and type of each approach for currency purposes.
• Safety pilot name — must be logged when flying under simulated instrument conditions.

Simulator & ATD Credit

Device	Currency Credit	IPC Credit	Training Credit (61.65)
Full Flight Simulator (FFS)	All 6 HITS	Full IPC	Up to 50% of required instrument time
Flight Training Device (FTD)	All 6 HITS	Full IPC	Up to 50% of required instrument time
Aviation Training Device (ATD)	All 6 HITS	Full IPC (with CFII)	Up to 20 hours (BATD: 10 hrs, AATD: 20 hrs)

Note: Aviator.NYC simulators qualify as ATDs, meaning you can complete all currency requirements and even a full IPC in our facility.

Safety Pilot Requirements

• Must hold at least a private pilot certificate with category and class ratings appropriate to the aircraft
• Must have a current medical certificate (or BasicMed)
• Must occupy a front seat with adequate visibility and access to controls
• The safety pilot is a required crewmember — they may log SIC time
• The aircraft must have dual controls

Complete Mnemonic Table

Mnemonic	Purpose	Letters
CRAFT	IFR clearance components	C — Clearance limit R — Route A — Altitude (initial and expected) F — Frequency (departure) T — Transponder (squawk code)
GRABCARD	Required IFR instruments and equipment (91.205(d))	G — Generator / alternator R — Radios (nav and comm appropriate for route) A — Altimeter (sensitive, adjustable) B — Ball (slip/skid indicator) C — Clock (hours, minutes, seconds) A — Attitude indicator R — Rate of turn indicator D — Directional gyro (heading indicator)
ARROW	Required documents in aircraft	A — Airworthiness certificate R — Registration R — Radio station license (international flights) O — Operating limitations (POH/AFM) W — Weight and balance
AVIATES	Lost communications procedure — route to fly (91.185)	A — Assigned (last assigned route) V — Vectored (last radar vector heading) I — Instructed (anything ATC told you to expect) A — as T — told to E — Expect (expected route/clearance) S — as filed (filed flight plan route)
NW KRAFT	IFR preflight weather briefing items	N — NOTAMs W — Weather (METARs, TAFs) K — Known ATC delays R — Runway lengths at destination A — Alternate requirements (1-2-3 rule) F — Fuel requirements T — Takeoff and landing distances
6 HITS	IFR currency requirements (61.57(c))	6 — Six instrument approaches H — Holding procedures I — Intercepting courses T — Tracking courses S — Six calendar months
IM SAFE	Personal fitness checklist	I — Illness M — Medication S — Stress A — Alcohol (8 hrs bottle to throttle, 0.04% BAC) F — Fatigue E — Eating / Emotion
PAVE	Risk assessment — identify hazards	P — Pilot (experience, currency, fitness) A — Aircraft (performance, equipment, fuel) V — enVironment (weather, terrain, airspace, airports) E — External pressures (schedule, passengers, mission)
DECIDE	Decision-making model	D — Detect the change E — Estimate the need to react C — Choose a desirable outcome I — Identify actions to take D — Do the necessary action E — Evaluate the effect
SAFETY	Risk management model	S — See the hazard A — Assess the risk F — Find options E — Evaluate options T — Take action Y — Your review (monitor and reassess)
ARCH	Lost communications — altitude to fly (91.185)	A — Assigned (last assigned altitude) R — as filed (altitude in flight plan) — whichever is C — Cleared (altitude ATC cleared you for) — the H — Highest of the three for each segment
DEPARTS	Departure briefing items	D — Departure procedure (ODP/SID) E — Engine failure plan P — Procedures (special/non-standard) A — Altitude restrictions R — Routing (initial climb, heading, fixes) T — Takeoff minimums S — Special notes (NOTAMs, TFRs, hazards)
A TOMATO FLAMES	Required VFR day instruments and equipment (91.205(b))	A — Airspeed indicator T — Tachometer O — Oil pressure gauge M — Manifold pressure gauge (if applicable) A — Altimeter T — Temperature gauge (liquid-cooled engines) O — Oil temperature gauge F — Fuel gauge L — Landing gear position indicator A — Anti-collision lights M — Magnetic compass E — ELT S — Seatbelts
FLAPS	Night VFR required equipment additions (91.205(c))	F — Fuses (spare set or circuit breakers) L — Landing light (if for hire) A — Anti-collision lights (beacon/strobes) P — Position lights (nav lights) S — Source of power (adequate electrical)
DV MONA	Magnetic compass errors	D — Deviation V — Variation M — Magnetic dip O — Oscillation N — Northerly turning error A — Acceleration error
D.E.P.S.	VOR receiver check methods	D — Dual VOR check (±4°) E — Estimated position / airborne (±6°) P — Published ground checkpoint (±4°) S — Station / VOT (±4°)
AVE-F	Lost communications — altitude memory aid (alternate version)	A — Assigned V — Vectored to expect E — Expected F — Filed — fly the highest for each route segment

VOR — VHF Omnidirectional Range

• Frequency: 108.0–117.95 MHz (VHF band)
• Principle: Broadcasts 360 radials from the station. The receiver compares the phase of a reference signal with a variable signal to determine the radial.
• CDI sensitivity: 10° full-scale deflection (each dot = 2°)
• Accuracy: ±1° for the station, receiver must be within ±4° for IFR use

VOR Service Volumes

Class	Altitude Range	Distance
Terminal (T)	1,000 ft – 12,000 ft AGL	25 NM
Low (L)	1,000 ft – 18,000 ft AGL	40 NM
High (H)	1,000 ft – 14,500 ft AGL 14,500 ft – 18,000 ft 18,000 ft – 45,000 ft 45,000 ft – 60,000 ft	40 NM 100 NM 130 NM 100 NM

VOR Receiver Checks — D.E.P.S. (14 CFR 91.171)

IFR flight requires a VOR check within the preceding 30 days:

Method	Tolerance
D — Dual VOR check (compare two VOR receivers)	±4° between the two
E — Estimated position (airborne checkpoint)	±6°
P — Published ground checkpoint	±4°
S — Station (VOT — VOR test facility, 360° or 0°)	±4°

Log: Date, place, bearing error, and signature.

VOR MON Program

The FAA's VOR Minimum Operational Network (MON) ensures that even as VORs are decommissioned, enough remain so that any point in the CONUS is within 100 NM of a VOR. If GPS fails, pilots can navigate using the MON to reach an airport with a VOR or ILS approach. MON airports are charted with a special symbol.

VOR Limitations

• Line of sight: VHF signals require line-of-sight; terrain and distance limit range
• Scalloping: Course roughness due to reflected signals near mountains or structures
• Cone of confusion: Unreliable signals directly above the station

DME — Distance Measuring Equipment

• Frequency: 962–1213 MHz (UHF band), paired with VOR/ILS frequencies
• Principle: Aircraft interrogates the ground station; station replies. The receiver measures round-trip time to compute distance.
• Slant range: DME measures the straight-line distance from aircraft to station, not horizontal distance. The difference is significant when close to the station at high altitude. Rule of thumb: slant range error is negligible when distance is at least twice the altitude (in thousands of feet).
• Accuracy: ±0.5 NM or ±3% of distance (whichever is greater)
• Groundspeed: DME provides groundspeed readout when tracking to or from the station

DME Service Volumes

DME service volumes match the co-located VOR (Terminal, Low, or High).

NDB — Non-Directional Beacon

• Frequency: 190–535 kHz (LF/MF band)
• Receiver: ADF (Automatic Direction Finder) — the needle points toward the station
• Bearing formula: MB = MH + RB (Magnetic Bearing to station = Magnetic Heading + Relative Bearing)

NDB Service Volume Classes

Class	Power	Range
Compass Locator (LOM/LMM)	<25 watts	15 NM
MH	<50 watts	25 NM
H	50–2000 watts	50 NM
HH	>2000 watts	75 NM

Compass Locator

A low-power NDB installed at the outer marker (LOM) or middle marker (LMM) of an ILS approach. It helps pilots locate the marker beacon and transition to the approach. Identified by a two-letter Morse code identifier.

NDB Limitations

• Night effect: Sky wave interference at dawn/dusk causes needle oscillation
• Terrain effect: Signal bends around mountains and coastlines
• Thunderstorm effect: ADF needle points toward electrical discharges
• Bank error: ADF antenna senses incorrectly when aircraft is banked