A chase plane is an aircraft

Aircraft

An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

 that "chases" another aircraft, a spacecraft or a rocket during flight. Safety can be one function of a chase plane; others are to photo or video the target vehicle, or to collect engineering data from it. Chase planes may be used during test flights, and for many years the best way to ensure the safety of an experimental aircraft

Experimental aircraft

An experimental aircraft is an aircraft that has not yet been fully proven in flight. Often, this implies that new aerospace technologies are being tested on the aircraft, though the label is more broad....

 was to fly alongside it and observe the flight.

This second airplane's crew would keep a constant lookout for problems with the test aircraft, and if problems did arise they would provide warnings and critical information to the test aircraft's crew and to the mission controllers on the ground. They would also monitor the surrounding airspace for other aircraft that could pose a hazard to the flight, either by straying into their flightpath or threatening any kind of secrecy associated with that particular project.

Since the early days of USAF testing, the chase aircraft have been manned by test pilot

Test pilot

A test pilot is an aviator who flies new and modified aircraft in specific maneuvers, known as flight test techniques or FTTs, allowing the results to be measured and the design to be evaluated....

s just like the experimental planes they accompany, providing a common language and bond between test pilot and observer. The experienced pilots in the chase aircraft could guide distressed test aircraft down to safe landings should experiments go wrong.