737 MAX Navigation Questions

In a typical dual flight management computer (FMC) arrangement, what generally happens if one FMC fails?

• The remaining FMC can continue to drive both control/display units, so navigation and performance computation isn't entirely lost
• Both control/display units go completely blank with no navigation function at all
• The aircraft immediately loses all electrical power
• The APU must take over the failed FMC's function

Explanation: A dual-FMC arrangement is normally set up so the surviving computer can supply both flight deck displays/control units, meaning the crew retains flight management and performance computation (with reduced redundancy) rather than losing the function entirely if one FMC fails.

Flight Management - Dual FMC Architecture

Why does an inertial reference system (IRS) generally need a longer alignment time at higher latitudes?

• The system aligns partly by sensing the Earth's rotation, and the horizontal component of that rotation rate it can sense decreases toward the poles, making alignment slower and less precise there
• Higher latitudes have less gravity, which slows the alignment process
• IRS alignment time is completely unrelated to latitude
• Alignment is actually faster at higher latitudes due to colder ambient temperatures

Explanation: Part of how an IRS aligns itself is by sensing the horizontal component of the Earth's rotation to determine true north; that horizontal component shrinks toward the poles (it's greatest at the equator and zero exactly at the poles), so alignment generally takes longer, and can be less precise, at higher latitudes.

Flight Management - IRS Alignment

Why is a standby magnetic compass still retained on modern flight decks despite extensive digital navigation systems?

• It provides a heading reference that works independently of all aircraft electrical power and computer systems, as an ultimate backup
• It is kept purely as a historical decoration with no operational function
• It is more accurate than any digital heading system and is used as the primary reference
• It is required only for aircraft that never fly over water

Explanation: A standby magnetic compass needs no electrical power or computer processing to work, making it the one heading reference that survives a total loss of aircraft electrical power or navigation computers - which is exactly why it's retained as a simple, independent last-resort backup rather than being removed once digital systems became standard.

Flight Management - Standby Magnetic Compass

Why might an aircraft carry more than one independent inertial reference system (IRS/ADIRU) unit?

• Redundancy - if one unit fails or its output becomes suspect, the others can continue supplying position/attitude data, and cross-comparing them helps catch a fault in any single unit
• Multiple units are fitted only to reduce total weight
• A single IRS unit is physically incapable of producing any usable output
• Extra units exist purely as unused spares with no active monitoring function

Explanation: Carrying multiple independent inertial reference units means the aircraft isn't dependent on any single one - if a unit fails, others can continue supplying the needed data - and continuously comparing their outputs against each other helps the system detect if any individual unit's data has become unreliable, rather than trusting one source blindly.

Flight Management - Multiple IRS Redundancy

Why might an automatic direction finder (ADF), an older style of radio navigation receiver, still be retained as a backup on some aircraft?

• It can still be used with widely available non-directional beacons (NDBs), which remain in use in some regions and for some approach procedures even as newer navigation methods have become primary
• ADF is required to be the primary navigation method on all modern aircraft
• ADF equipment provides more accurate position information than GPS
• ADF has no remaining operational use anywhere in the world

Explanation: Even though satellite and area navigation systems are now primary in most regions, non-directional beacons (NDBs) - which an ADF receiver is built to use - remain in service in various parts of the world and for some published procedures, so retaining ADF capability keeps the option of using those facilities available as a backup or where required.

Flight Management - ADF as a Backup Navigation Aid

During an approach, what does a lateral deviation (course deviation) indication on the primary flight display generally show the pilot?

• How far the aircraft is currently offset to the left or right of the intended lateral course, helping the pilot make timely corrections to stay on track
• The aircraft's current airspeed relative to the target approach speed
• The remaining distance to the destination in nautical miles
• The current outside air temperature

Explanation: A lateral deviation indication gives the pilot a continuously updated, at-a-glance picture of how far off (left or right) the aircraft currently is from the intended course, letting corrections be made promptly and smoothly rather than needing to compare raw position data to a chart to figure out the current offset.

Flight Management - Lateral Deviation Indication

What does 'cross-track error' generally represent on a moving-map style navigation display?

• The current lateral distance between the aircraft's actual position and the intended track between two waypoints
• The aircraft's altitude above the terrain directly below it
• The remaining flight time to the destination
• The difference between true and magnetic heading

Explanation: Cross-track error is simply how far off to one side the aircraft currently is from the straight line (track) it's supposed to be flying between two waypoints - a moving-map display commonly shows this graphically so the pilot can see at a glance how far off course the aircraft has drifted and correct accordingly.

Flight Management - Cross-Track Error

What does a 'direct-to' function on a flight management system generally let the crew do?

• Command the aircraft to fly directly toward a selected waypoint from its present position, rather than continuing to follow the previously planned route sequence
• Automatically declare an emergency to air traffic control
• Bypass all fuel planning calculations entirely
• Force the aircraft to immediately climb to its maximum operating altitude

Explanation: A direct-to function lets the crew quickly command the aircraft to fly a direct course from its current position to a chosen waypoint, cutting across the previously planned route rather than continuing to follow it leg by leg - useful for a shortcut, a course change requested by ATC, or simply getting back on course after a deviation.

Flight Management - Direct-To Function