04 Navigation & PBN

What is the practical difference between a great-circle and a rhumb-line route?

A great circle is the shortest route between two points but normally requires changing track; a rhumb line maintains a constant track but is generally longer.

How are true, magnetic and compass directions related?

True corrected for variation gives magnetic; magnetic corrected for deviation gives compass.

What does wind change: aircraft heading, track, or both?

Wind changes the relationship between heading and track; the pilot/FMS changes heading to maintain the required track.

What is PBN fundamentally based on?

Specified navigation performance requirements for an operation, rather than prescribing a particular navigation sensor.

What is the key difference between RNAV and RNP specifications?

RNP includes on-board performance monitoring and alerting; RNAV does not require it.

What does the number in RNAV 1 or RNP 1 mean?

Lateral total system error of ±1 NM is expected for at least 95% of flight time.

How should RNAV 5, RNAV 1 and RNP 1 be thought of operationally?

RNAV 5 is mainly en-route; RNAV 1 and RNP 1 support more accurate terminal navigation, with RNP 1 additionally monitoring and alerting navigation performance.

What is the operational relationship between ANP and RNP?

The estimated actual navigation performance should remain within the required RNP; ANP greater than RNP means the required performance cannot be assured.

What are the four key navigation-performance concepts?

Accuracy, integrity, continuity and availability.

Why is integrity different from accuracy?

Accuracy describes how close the estimated position is to truth; integrity concerns timely warning when the navigation solution should not be trusted.

What navigation accuracy applies to the final segment of an RNP APCH to LNAV or LNAV/VNAV minima?

RNP 0.3.

What navigation accuracy normally applies to the initial, intermediate and missed-approach segments of RNP APCH?

RNP 1.

What is the practical difference between LNAV, LNAV/VNAV and LPV?

LNAV provides lateral guidance only; LNAV/VNAV adds approved vertical guidance, commonly Baro-VNAV; LPV uses SBAS to provide precise lateral and vertical guidance.

Does LPV use an ILS localizer or glide-slope transmitter?

No. The guidance is GNSS/SBAS based.

Why can LPV nevertheless feel similar to an ILS?

It provides highly precise lateral and vertical guidance with a decision altitude.

Why are correct QNH and temperature particularly important for Baro-VNAV?

The computed vertical path depends on barometric altitude, and temperature affects the relationship between indicated altitude and true geometric height.

What is RNP AR APCH?

An Authorization Required RNP approach using tighter containment and additional aircraft, crew, procedure and operator requirements.

How low can the navigation accuracy requirement become on an RNP AR APCH?

Depending on the segment and procedure, it may be as low as RNP 0.1.

Why does RNP AR require specific operational approval?

Tighter containment may require specialised aircraft capability, training, procedures, data management and monitoring.

What is an RF leg and why is it different from a normal turn?

A Radius-to-Fix leg is a precisely defined constant-radius curved path that the navigation system must follow.

Why should a direct-to clearance to a fix immediately preceding an RF leg normally not be accepted?

It can destroy the geometry required for correct RF-leg path capture.

What is the practical difference between fly-by and fly-over waypoints?

A fly-by waypoint allows turn anticipation before the fix; a fly-over waypoint must be crossed before the turn begins.

Why are path terminators important in an FMS database?

They define exactly how each procedure leg is to be flown, not merely where the next waypoint is located.

Why should a published procedure normally be loaded from the navigation database rather than manually constructed?

The validated database contains the designed waypoint coordinates and leg coding on which procedure containment and obstacle protection depend.

Why must the loaded FMS procedure still be compared with the chart?

To detect wrong procedure selection, unexpected coding or database discrepancies.

Why should crews avoid manually modifying waypoints in a coded PBN procedure?

The modification may alter the protected path or invalidate the assumptions of the procedure.

What is AIRAC and why does it matter to navigation databases?

Aeronautical information changes follow a standard 28-day cycle; an outdated database may contain obsolete procedures or coordinates.

Why is WGS-84 important in modern navigation?

It provides a common geodetic reference so procedure coordinates and aircraft-computed position use the same Earth model.

How does GNSS determine aircraft position?

By measuring pseudoranges to multiple satellites and solving for three-dimensional position plus receiver clock error.

Why are at least four GNSS satellites normally required for a full 3-D solution?

The receiver must solve latitude, longitude, altitude and its clock error.

What is RAIM and what does it provide?

Receiver Autonomous Integrity Monitoring uses redundant GNSS measurements to detect an unreliable navigation solution.

What is the relationship between RAIM and ABAS?

RAIM is a form of Aircraft-Based Augmentation System functionality.

What is the practical difference between SBAS and GBAS?

SBAS provides wide-area satellite-broadcast corrections and integrity; GBAS provides local ground-based augmentation around an aerodrome.

What is the European SBAS called?

EGNOS.

What is the difference between GNSS jamming and spoofing?

Jamming prevents or degrades reception; spoofing creates false GNSS-like information that may produce a misleading position or time solution.

Why can spoofing be more insidious than jamming?

The navigation solution may continue to look plausible instead of simply becoming unavailable.

What is the basic crew strategy when GNSS interference is suspected?

Cross-check independent navigation sources, identify unreliable information and follow aircraft/operator and ATC contingency procedures.

Why are inertial systems valuable during GNSS interference?

After alignment they are self-contained and do not depend on external radio signals.

What is the main limitation of pure inertial navigation?

Small sensor errors accumulate, so position error gradually increases with time.

What is sensor blending in a modern navigation system?

Combining GNSS, inertial and radio-navigation information to produce the best available position estimate.

Why is sensor independence important?

Independent sources allow erroneous or degraded navigation information to be detected rather than blindly accepted.

What is the key limitation of DME distance close to and above a station?

DME measures slant range, so vertical distance becomes a significant part of the indication.

Why does DME/DME navigation depend strongly on station geometry?

Poor intersection geometry magnifies range-measurement errors into larger position errors.

What does a VOR radial describe?

The magnetic bearing FROM the VOR station, independent of aircraft heading.

What is the purpose of raw-data cross-checking on a modern FMS aircraft?

To provide an independent reasonableness check of the computed navigation solution when suitable ground-based aids are available.

How closely should an aircraft normally track the centreline during RNAV/RNP operations?

Normal cross-track deviation should generally remain within about half the applicable RNAV/RNP value.

Why is a direct-to the FAF normally inappropriate on an RNP APCH?

The aircraft should be correctly established on the final approach track before the FAF; current EASA guidance does not accept direct-to the FAF.

When may a direct-to the IF be acceptable on an RNP APCH?

When the aircraft can be established on the final approach track at least 2 NM before the FAF.

When should an RNP APCH normally be discontinued because of navigation performance?

On navigation-system failure, excessive lateral/vertical deviation, or loss of required onboard monitoring and alerting unless an approved alternative capability or sufficient visual reference permits continuation.

Does loss of GNSS always mean immediate loss of all PBN capability?

No. A suitably approved multi-sensor system may retain the required RNP capability using other sensors.

What is the difference between loss of GNSS reception and loss of GNSS integrity?

Reception loss means the signal is unavailable; integrity loss means the system can no longer assure that the navigation solution is trustworthy.

Why must crews understand navigation-system reversion modes?

After sensor degradation the FMS may continue operating but at a lower navigation capability that may no longer support the planned procedure.