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Going Downhill
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Flight to Lower Elevations
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 By John Lorenz
Us macho and macha high-desert pilots get to feeling smug about flying at high density altitudes and in mountainous terrain, and about how pilots flying from less exalted elevations have to adjust to what we deal with regularly, albeit not always successfully. However, there are also important changes to be aware of when going from 9000-foot density altitudes down to the thicker soup they call ‘atmosphere’ at sea level.
For one, the nose-up attitudes that produce Vx, Vy, and cruise climb airspeeds are steeper than those we’re used to, since the engine, now sucking in as much as 20% more oxygen, is producing up to 20% more power and is capable of dragging the airframe up a steeper hill. Lowland pilots who are accustomed to such steep climb angles get caught nose high and slow, closer to a stall than they realize, when trying to out-climb obstacles or rising terrain at unaccustomed high density altitudes. But a pilot going the other way, who is used to the relatively shallow climb attitudes and who is trying to out-climb trees at the end of a sea-level runway, may not be using the full potential of the airplane.
The additional power near sea level means you may have to hold the brakes harder during runup and that marginal brakes may not even hold completely, so watch for forward creep while checking the mags. Additional power also means that fuel consumption goes up: the eight gallons per hour that the Lycoming O-320 has been burning rain or shine, night or day, taxi or full-power climb, may jump to 10 gallons per hour. That, combined with slower true airspeeds, results in significantly reduced range in terms of both hours aloft and miles over the ground, a potential ‘gotcha’ with serious consequences.
Maintaining a proper airspeed is critical on final, and a 65-knot approach should be flown at 65 knots regardless of density altitude. Unfortunately, near the ground much of our perception of speed is based on the visual impression of forward speed over the ground. Sea-level pilots get into trouble because they are used to seeing the ground go by more slowly on short final than is required to maintain a safe airspeed at higher density altitudes. Therefore, they sometimes let airspeed bleed off to dangerous conditions in order to maintain the accustomed impression of speed over the ground. On the other hand, pilots who are used to seeing the ground go by faster on final are prone to flying final approach too fast at sea-level airfields, since otherwise it appears to us that the aircraft is loafing along at dangerously slow speeds. The main problem with a fast approach is that many sea-level runways are relatively short: with full power available from engines, runways at lower altitudes don’t have to be as long as those in the mountains. 2000-ft runways, bounded by FAA-approved 50-ft trees, are not uncommon in Ohio. Arriving in the flare with excess airspeed leads to floating and possible runway over-runs. A pitot tube can’t see the ground, and it responds to changes in density altitude the same way lift from the wing does, so be aware of the airspeed indicator on final and use the normal approach airspeed.
Haze and limited visibility are more common at lower altitudes, and the numerous small towns often seem to have been cloned. It takes some mental re-adjustment for pilots who are used to navigating by heading for a distinctive mountain range 50 or 75 miles away to navigate successfully where "good" visibility is only 5 to 10 miles, all roads run north-south/east-west, and where everything is flat and green. There are also more tall radio towers where the population is denser. Towers aren’t plotted on most GPS databases, so plan to stay high or else keep track of their positions on a sectional map and anticipate them.
High humidity is common at lower altitudes whether or not it’s hazy, so carburetor icing and fuel tank contamination from condensation are both more likely. The first reaction to pulling the carb heat if ice is present in the venturi will be for the engine to run more roughly as it melts and ingests the ice, and it takes willpower to let the process continue for a minute or two until the engine runs better.
Management of the mixture is also different. Lean for taxi but remember to go full rich before takeoff and again before landing. A go-around with a leaned mixture might cause the engine to balk; at a minimum, it would be performing with less than maximum power. It can also cause engine damage.
The grass airfields that are more common at low altitudes make dirt and gravel strips of the desert look like ragged cousins, but grass can be treacherous. It’s slippery when wet, diminishing directional control and virtually negating braking. Tall grass can add drag and extend a takeoff roll, so a soft-field takeoff technique is appropriate.
Familiarity with flight at high density altitudes doesn’t automatically over-qualify us for flight at sea level. Aviation, for all its numbers, benefits from a bit of flexibility. |
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