Basic airspeed concepts: which of the following statements is correct regarding how aircraft speed is referenced?

Introduction / Context:
Aviation distinguishes between airspeed and groundspeed. Airspeed determines aerodynamic performance (lift, stall margin), while groundspeed affects navigation and time en route. The question probes correct terminology and the influence of wind on these speeds.

Given Data / Assumptions:

• Wind can be headwind or tailwind.
• Cruising speed is an operational target (often an airspeed), not a reference frame.
• Groundspeed is velocity over the ground.

Concept / Approach:
Airspeed is the aircraft’s speed relative to the air mass. Groundspeed equals airspeed plus or minus wind component along the track (positive for tailwind). “Cruising speed” is a chosen airspeed regime (e.g., economy cruise) rather than a reference frame term. Therefore, only the statement defining airspeed correctly addresses the reference to wind/air.

Step-by-Step Solution:
Define airspeed: speed relative to the air → correct.Identify groundspeed: speed relative to the ground (not “cruising speed”).Note tailwind effect: groundspeed increases while airspeed can remain the same; statement (c) confuses terms.Hence, only option (b) is correct.

Verification / Alternative check:
Pilot references: IAS/TAS determine performance; GPS/INS gives groundspeed for navigation. Wind triangle shows vector addition of wind to airspeed to obtain groundspeed.

Why Other Options Are Wrong:

• (a) Mislabels groundspeed as cruising speed.
• (c) Tailwind increases groundspeed relative to airspeed; the statement is misleading.
• (d) Collectively incorrect because (a) and (c) are not correct.

Common Pitfalls:

• Assuming “cruise” always means a fixed groundspeed; it is typically an airspeed setting.

Final Answer:
The speed of the aircraft relative to the surrounding air mass (wind) is called airspeed.