Premium on Safety - Issue 33, 2019 - 7

QUIZ ANSWERS
Questions on page 3

1. (B) Oxygen is a heavier element so it sinks to the surface of the
earth. Most of the earth's oxygen is below 35,000 feet. Other
elements are lighter so they are higher in the atmosphere. Aircraft
engines burn oxygen. When there is less oxygen the engines use less
fuel and can therefore be more efficient, to a point. When there simply
isn't enough oxygen left in the atmosphere jet engines flame out.

2. (B) The standard atmospheric temperature (15 C, 59 F) and pressure
(1013mb, 29.92 in) is a baseline for calculating aircraft performance
throughout a range of atmospheric conditions. Most days, most places
vary from standard, and pilots must calculate their expected aircraft
performance based on the variation away from ISA (international
standard atmosphere).

3. (A) True. As the air becomes less dense (higher density altitude),
you should consider leaning your engine for maximum RPM per the
POH recommendations. A full-rich mixture in a high density altitude
environment means your airplane will not have the appropriate
fuel-air mixture to develop full power for takeoff. Some operating
handbooks recommend deploying the first notch of flaps, but full
flaps will create more drag than lift during departure.

4. (A) True. Humidity affects the way an airplane flies because of the
change in pressure that accompanies changes in humidity. As the
humidity goes up, the air pressure for a given volume of air goes
down. H20 is lighter than O2 or N2, the primary components of air.
This means the wings have fewer air molecules to affect as they are
pushed through the airmass. Fewer molecules = less lift. The other
problem is that jet engines do not like humidity either. Jet engines are
built for cold, dry air, and humid air has fewer oxygen molecules to
burn per unit volume. Therefore the engine combusts a little bit less
and puts out slightly less thrust.

5. (F) All of the answers are correct. Engine air intakes are often just a
foot or two from the ground, so during the takeoff roll they may be
ingesting air that is significantly hotter than what the AWOS/ASOS
is sensing. Performance charts are computed by professional test
pilots flying a brand new aircraft from the factory, and your engine is
probably not new, and odds are your skills aren't quite up to the level
of the test pilot's. Under-inflated tires will not accelerate as quickly,
therefore your ground roll will be longer than calculated, and any
mistake on weight and balance calculations will impact performance
as well. To prevent these kinds of awful surprises on takeoff, make
a note to operate out of high elevations or on hot, humid days with
reduced fuel and passenger loads, and overestimate your necessary
runway length for takeoff (and balance field if that applies) by 30
percent or better. And check your tire pressures!

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Premium on Safety - Issue 33, 2019

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