GPS numbers and aircraft instruments are different

Use the label before the number

A moving map can show several accurate values that still answer the wrong question for the task at hand. The FAA Pilot's Handbook navigation chapter distinguishes the intended course, the direction the aircraft points, its actual track over the ground and its speed over the ground. Wind is one reason these values differ.

Three distinctions worth rehearsing

• Ground speed versus airspeed: a GPS ground-speed value describes movement over the earth. It does not replace the aircraft's airspeed indication for aircraft operation.
• Track versus heading: the path being made over the ground is not necessarily the direction the nose points. Check whether a displayed direction uses true or magnetic north before comparing numbers.
• GPS altitude versus indicated altitude: a geometric satellite-derived height and an aircraft pressure-altimeter indication come from different measurements. They are not interchangeable simply because both use feet.

The FAA flight-instruments chapter explains pressure-based altitude and airspeed indications. Review the applicable aircraft instruments and procedures with your instructor rather than trying to resolve a cockpit disagreement by choosing whichever tablet number looks more convincing.

Check source and freshness

During a ground rehearsal, locate the app's position-source name, age and loss-of-position indications. Write a simple note describing which information you will no longer treat as current after a source failure. A previously good number that remains visible is not automatically a fresh measurement.

FLAL labels estimated position separately from a current GPS fix. That estimate is not a newly measured location. It must not be used to establish obstacle clearance, instrument-procedure compliance or the current aircraft position when the actual source is unavailable.

An exercise for the next lesson

Ask your instructor to review a still screenshot of your chosen flight display. Name each field, its source, units and intended use. Then explain which aircraft instrument answers a different question. Do this before flight, when there is time to discuss ambiguities.

The aim is not to make the tablet imitate the panel. It is to know exactly where supplemental map information helps and where it does not.