PLT151: Buoyancy In Lighter Than Air Flight
What code PLT151 covers, why the FAA tests it, and how to study Buoyancy In Lighter Than Air Flight before your checkride.
Recall airship - buoyancy
What this means for your checkride
Lighter than air flight relies on buoyancy, the upward force a surrounding fluid exerts on an object immersed in it. The net upward force equals the weight of the air the craft displaces, so the craft floats when it displaces enough air to equal its own weight. A hot air envelope reaches this state by heating the air inside so it expands and becomes less dense than the surrounding air, producing positive buoyancy.
Buoyancy is the fundamental principle behind every lighter than air craft, and the displaced weight relationship explains why heating or the choice of lifting gas changes lift.
The craft does not rise because hot air pushes down. It rises because the ambient air it displaces weighs more than the craft and its lifting gas.
Where to study it
A code on your test report is an area of deficiency. An authorized instructor must review it with you and endorse that review before you take the practical test, and the examiner may revisit it during the oral. Study it until you can explain it out loud, not just recognize the answer.