In
meteorological terms there is what is called “density altitude.” In simple
terms it relates to temperature verses atmospheric pressure. For an
airplane/helicopter pilot this difference has a definite effect on take offs
and landings. A miscalculation can result in serious, if not fatal,
consequences.
Regardless of the place
of takeoff if the temperature is above normal the actual altitude is higher
than the physical location of the field or airport as far as air is concerned. As
a result the hotter and humid of the temperature the “density altitude”
increases accordingly. The result is the air has less “lift” in it than if the
temperature was cooler. Consequently an airplane needs more runway and speed to
get off the ground.
Lift is generated by the
airfoil of the wing and is drastically affected because of the “thin” air. Propellers
are also considered “wings” in themselves because of their airfoil. The wing
generates vertical lift and the propeller provides horizontal thrust. As a result
of the thin air caused by the higher temperature and humidity, this causes less
“bite” than a propeller provides at a normal temperature. To compensate for all
of this the aircraft load has to be reduced or you sit on the ground and wait
for evening time and cooler air.
As an example; a few
years ago at Arizona, the Phoenix International Airport was shut down when the
temperature rose over 120 F. Since the temperature exceeded the takeoff charts
the jets sat on the ground awaiting a cooling evening. A miscalculation could
have very unhappy results!
For an “ag pilot”
considering density altitude is a must. Unfortunately pilots flying agriculture
airplanes don’t have charts to compute takeoff distances. They rely only on
their experience.
I recall flying a Thrush
Commander in Central California a few years back. The Thrush is a wonderful
airplane equipped with a 650, 800 or even a 1200 horsepower radial engine, a
44-foot wingspan and a 400-500 gallon hopper. It is capable of carrying a full
load weighing a ton of material or more depending on the engine installed.
Where I was flying it was all night spraying and it was nothing to takeoff with
a full load of chemical off a half mile dirt strip. I have frequently been asked
“why fly at night?” The answer is simple. At night there is less wind and also
the temperature is much cooler than daytime. Density altitude.
There was another time I
was flying out of Casa Grande, Arizona during the hot summer. Even at night the
temperature reached over 90 degrees. Flying the same model of Thrush as I did
in California I had to reduce the load down to 150 gallons. The airplane
couldn’t take out any more and fly. Even cutting back on the load the air was
so dead the plane flew like a lead sled and making turns I was virtually at a
near stall. This could prove to be a chancy way to make a buck as one pilot
remarked. And this was flying at near sea level.
Sometime later I had the
opportunity to fly out of American Falls, Idaho. Again it was summer and we
were hampered by high temperature and altitude. American Falls, I recall, was at
almost 5,000 feet elevation above sea level. Consequently at this higher
physical altitude the density altitude was much higher—much as Phoenix. The
result was I had to cut back on the load and allow for more takeoff distance. Since
there was no way we could fly out of our home field we elected to fly off a
nearby paved airport with over a mile long runway. It was miserable flying!
I was back flying a
Thrush and the other pilot was flying a Grumman “B” model Ag Cat. The “Cat” is
also a wonderful agricultural by-wing airplane that had a smaller hopper for
chemical. The pilot, who went by the initials “TD,” was building experience. He
didn’t have a lot of “ag” flying time and had just started flying the “Cat.”
Loading the airplane I
told my son Dave who was filling the hopper to cut back on the load. Talking
with TD we agreed to meet at the field we were supposed to spray and I would
take off first. After Dave was finished I also checked the fuel tanks to make
sure I carried just enough gas. There is no sense in filling the gas tanks with
un-needed gas since that would amount to more weight. Finally I climbed into
the cockpit, put on my crash helmet (which resulted in immediate sweating
causing sweat to run down into my eyes!), fired up the engine and taxied out to
the runway. Checking all the engine gauges and running the prop through to make
sure it changed pitch correctly I lined up for takeoff.
The engine responded
smoothly and the airplane sluggishly started to move down the pavement. It
seemed like a lifetime to get up to speed and the controls felt limp. Finally
the tail came up but I was already almost half way down the strip. The engine
was screaming and I was quickly running out of sufficient runway when all at
once there was firmness in the stick and I was able to nurse the airplane into
the air. As I passed the end of the runway there were houses ahead and I didn’t
dare pull back any power. I screamed across the roofs at about 50 feet and then
there was an open field in front of me and I was able to get everything under
control.
When I got to the field
where TD and I were to rendezvous he was nowhere to be seen. I circled the field
a few times burning off fuel. With less fuel the plane started handling better
and finally I said to myself “to heck with it,” and went ahead and sprayed the
field.
Returning to the airport
I landed and taxied to our loading area. Dave came over and I asked him where
was TD? He responded that he didn’t know and that he had taken off just behind
me. Getting out of the airplane and taking off my crash helmet I wiped the
sweat from my eyes and got a drink of water. Just then a taxi cab arrived and
getting out of the car was TD with helmet in hand. He looked a mess. His
overalls were filthy and torn and he was covered with dirt.
“What happened?” I asked.
“I crashed; what the hell did you think I did,” he said in disgust. Then the
story came out. He neglected to take into account the temperature and had Dave
fill his hopper to full. He saw me take off and head over the houses to the
field and figured If I could do it so could he!
As the plane ran down the
runway trying to get into the air he barely was able to pull the plane off the
ground when he immediately found himself in trouble. To compound things he made
another mistake—he started to pull the power off to cruise setting and
immediately stalled and crashed into a pig pen! The next thing he knew he was
sitting in the wreckage inside the pen surrounded by a bunch of curious pigs.
But that wasn’t enough.
That evening Paul Harvey, a renowned national radio broadcaster and pilot
himself, put poor TD’s unfortunate incident of landing into a pig pen on his
newscast. This newscast, which was heard in all 50 states, was known as “…and
now you know the rest of the story.”
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