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June 1, 1991
RICHARD L. COLLINS
Summer means great flying vacations. The weather is usually good, and the days are long. Packing up the airplane and heading out can be a special treat. This is especially true if you head for new territory. Taking a couple of weeks to explore the western United States can be a fine experience for an Easterner. If there is any drawback, it relates to not making peace with some special factors that affect airplanes in warm weather, especially at higher altitudes.
Density altitude is something that we all learn about in training. It is altitude corrected for temperature and is the only altitude that is understood by the airplane. When the specs show a service ceiling, for example, that ceiling is density altitude. A Cessna Skyhawk has a service ceiling of 13,100 feet at its maximum takeoff weight. The density altitude at Colorado Springs on an 80-degree day is almost 9,000 feet. That doesn't leave much leeway, especially if you are thinking about vaulting the mountains to the west. One of the keys to operating an airplane like a Skyhawk in the West is flying early or late in the day, when the temperature is lower. There is a big comfort factor here, too. Thermal turbulence often reaches into the high teens or low 20s in high country on a hot summer day. It is virtually required that you not plan on maximum-takeoff-weight operations unless dealing with some of the lower routes through the mountains.
Trying to make an airplane do something that it won't do is a leading cause of accidents. In one study of fatal stall/spin accidents, more occurred during the initial climb than in any other phase of flight. Most of the airplanes involved were those without an excess of horsepower. An average scenario might see a heavily loaded airplane departing from a relatively short field on a hot day. That is a tough combination, one that requires planning plus compromises.
The first part of the plan hinges on conservative use of the pilot's operating handbook. For an example, let's say that we will be flying a 1975 150-horsepower Skyhawk from a 2,200-foot-long grass strip, with 50-foot-high trees 300 feet from the end of the runway. The elevation is 2,500 feet, the wind is calm, and the temperature is 80 degrees Fahrenheit.
Cessna used a correction for temperature variation from standard in the 1975 POH. The admonition was to increase takeoff distances by 10 percent for each 25 degrees F above standard. Because it is 30 degrees above, we'll add 12 percent to the distances shown. The standard-temperature ground roll is 1,040, and the distance over 50 feet is 1,910 feet. The correction for temperature increases the distance to clear the obstacle to 2,139. There's a further 7-percent addition for the grass runway, which increases the distance over the obstacle to almost 2,300 feet. That means that the airplane will be at treetop level when 300 feet from the trees. A little math shows that this distance would be covered in just over three seconds. Pucker time. The numbers in the handbook reflect a new airplane perfectly flown by a test pilot. Liftoff is precisely at the speed shown, and the climb is at the best- rate-of-climb speed. There is no turbulence. Even with all that, you can bet that the test pilot would not attempt the feat were the 50-foot obstacle a brick wall.
Building margins is what turns dangerous operations into safer ones. In this example, there is only one way to build in a margin. The takeoff weight of the airplane would have to be reduced by leaving behind passengers, baggage, or fuel. Prudent pilots leaving small airports have been known to shuttle passengers to a larger airport one at a time. Or making a short hop to a nearby larger airport for fuel might do the trick.
Just cutting the load by 300 pounds, for a takeoff weight of 2,000 pounds, cuts the distance required to take off and climb to 50 feet to just under 1,600 feet in these conditions. That is a lot more margin. The simple fact is that maximum takeoff weight is just that — a maximum — and there are operations that are hazardous at that weight. This example is one of them.
Rules of thumb abound on margins for takeoff. One suggests that you total the ground roll for the takeoff as well as for a landing to calculate a minimum runway length. In theory, this would allow a successful abort of the takeoff if acceleration wasn't good enough to reach liftoff speed by a given point. In the example, though, this doesn't help much because the combined takeoff roll and landing roll distance comes to under 1,800 feet. The Skyhawk's limitation in these conditions is related more to a low rate of climb after takeoff than a prolonged ground run. Regardless of the airplane you fly, you can get a feel for this by comparing the listed ground run with the distance to clear a 50-foot obstacle. The calculated ground run for this Skyhawk example is about 1,250 feet; it takes almost an additional 1,000 feet to climb 50 feet, which is not exactly what you would call a spirited climb.
Beware using flaps, too. The 1975 Skyhawk manual states that 10- degree flaps may be used for takeoff and that this will shorten the ground run by about 10 percent. This advantage is lost in the climb to 50 feet, though, because flaps create drag as well as lift. The book further states that flaps are not recommended for high and hot takeoffs.
Perhaps the best margin is to consider that you want to clear the obstacles by at least 50 feet and want to also add 25 percent margin to the distance required to account for error, low compression, and pilot technique.
Judging acceleration is an important part of hot and high flying technique. After doing all the careful calculations on the ground roll, take a look at the runway. Where on the runway should liftoff occur? If the margins are not generous, any takeoff run that exceeds the calculated distance will lead only to a flirtation with treetops. Be suspicious.
An airline accident some years ago serves as a good illustration on acceleration, even though it was related more to cold than hot. The crew calculated a rotation speed for takeoff of 153 knots. According to calculations, this speed should have been reached in 5,700 feet, and the aircraft should have actually lifted off 6,600 feet down the runway at a speed of 163 knots. The runway was 10,900 feet long. The margin was there. The crew said acceleration was normal up to about 130 to 135 knots, where it flattened out. Actually, the airplane reached a speed of only 153 knots, and that was 10,400 feet down the runway. Of course it didn't fly. It finally stopped 3,400 feet past the end of the runway. What happened? The brakes were locked throughout the takeoff. The crew didn't notice this early on because the runway was covered with ice. The airplane was literally trying to slip- slide away. When some speed was gained, the main gear tires blew out, and the airplane was trying to accelerate while running on the rims. Even though the pilots said initial acceleration seemed okay, it took the airplane 20.8 seconds and 3,200 feet longer than normal to reach V1, which was 138 knots. The moral is to always have a plan to judge acceleration and to abort the takeoff if it isn't normal.
If the flight is IFR, you have to be aware of minimum climb requirements. Terrain and obstacle clearance on an IFR departure assume a climb rate of 200 feet per nautical mile traveled.
The Skyhawk will, at maximum takeoff weight, climb 250 feet per nautical mile at the best-rate-of-climb speed in the conditions used in the example. A tailwind in the climb would decrease that number because of the increase in groundspeed. A sub-par engine, turbulence, or downdrafts might do likewise. Check for special IFR departure procedures, included on the chart in the case of Jeppesen or flagged with "T" dropped out of a black triangle at the bottom of NOS charts. These might include special climb routes or procedures or a requirement for climb greater than 200 feet per nautical mile.
If the departure is from an airport located in uncontrolled airspace, avoiding terrain and obstacles is strictly up to the pilot until the aircraft enters controlled airspace. Procedures do not take into account obstacles that don't impinge on controlled airspace, and with a 700-foot floor, there can be a lot of hurdles to clear before entering controlled airspace. A thorough survey of the climb path, on a sectional or on the approach chart for the airport, is an important part of any IFR departure from an airport in uncontrolled airspace — especially when the climb is in question as it might be at a high density altitude.
Thinking in terms of climb as feet per mile is not how we normally do it, but that is how the ATC system is designed. We can put this to good use when planning a flight in mountainous areas. If it is 30 miles from the airport to the ridge and you have to climb 6,000 feet to clear the ridge by a safe margin, that's 200 feet per mile. If the climb is at 60 knots, that's 200 feet per minute. Ninety would be 300 fpm, and 120 would require 400 fpm. Remember, though, that the higher the airplane climbs, the lower the rate in fpm as well as feet per mile.
While doing the sums tells us that using lower than maximum allowable weights is an important part of maintaining margins when it gets hot, especially at higher elevations, the accident records show that pilots often go the other way and fly their airplanes at weights in excess of the maximum takeoff weight. Some pilots do this on a regular basis and contend that "she flies just fine with everything you can stuff in the cabin and the tanks." The truth is she flies just fine right up to the moment of impact. Disregarding weight and center of gravity limitations results in a lot of accidents — 213 in one five-year period studied by the National Transportation Safety Board. An illustration that weight- or CG-related accidents are usually serious is found in the fact that 273 people died and 115 were seriously injured in these accidents.
Accidents in which the airplane was overloaded often occur at relatively high density altitudes. One pilot crashed while attempting a takeoff in an overloaded airplane at a density altitude of 9,800 feet. Another overloaded airplane was destroyed on an uphill departure from a short strip at a density altitude of 3,000 feet. Double jeopardy is best left to television.
The best way to handle high density altitudes is to pad your plans by being flexible on weight, fuel, and time of departure. This could also mean operating a six-seat airplane as if it were a four-seater; a four-seater like a two-place. Build in a margin that will help keep summertime sweating to a minimum.
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