What can pilots use to adjust their navigation if they are aware of magnetic variation?

Pilots can use the compass heading to adjust their navigation effectively when they are aware of magnetic variation. Magnetic variation, also known as magnetic declination, refers to the angle difference between true north and magnetic north. This can affect navigation as pilots rely on compass headings to determine their flight path.

When pilots know the magnetic variation for their location, they can make the necessary adjustments to their compass readings to convert them to true headings. This ensures that their navigational calculations are accurate, allowing for more reliable flight planning and navigation.

The other options listed do not serve the same purpose in the context of adjusting navigation for magnetic variation. Airspeed indicators, altitude transponders, and GPS systems provide different types of information crucial for flying, but they do not directly deal with the magnetic variation that affects compass readings.