Don't fight the charts

The standard atmosphere chart shows how air density is affected by altitude, barometric pressure, and temperature.

Do you calculate your airplane's takeoff, climb, cruise, and landing performance before each flight? What could happen if you don't? The outcome of a flight could vary from surprising--you used more runway than usual after landing 10 kt faster than the recommended speed on final--to disastrous.

Consider this report from the National Transportation Safety Board. Shortly before 1 p.m. on October 23, 1999, a Cessna 172R with three persons aboard departed the Pitkin County airport in Aspen, Colorado (the accident report states that field elevation is 7,815 feet above mean sea level [msl]; other publications specify 7,820). After takeoff, the aircraft climbed eastward straight up the valley toward Independence Pass. Minutes later, a hiker saw the aircraft crash 1.5 miles short of the crest of Independence Pass "flying slowly, in a climb, with a nose-high attitude." The pilot died, and two passengers--both certificated pilots--were seriously injured. The three pilots had a cumulative 445 flight hours, mostly in Cessna 172s.

The temperature at Aspen was 62 degrees Fahrenheit, 20 degrees above "standard." Elevation of the crash site was 11,948 feet msl. The crest of the pass was at 12,095 feet msl, and the density altitude at Independence Pass was 14,100 feet msl.

Under standard conditions, performance charts showed that it should have taken the aircraft 26 miles to clear the elevation of the pass. But the conditions were not standard.

The NTSB accident report stated that, under prevailing conditions (20 degrees hotter than standard), aircraft performance charts required a climb distance of 28.6 miles for the aircraft to have cleared Independence Pass. The crash site, however, was only 19 miles east of the Aspen airport, 9.6 miles short of the distance required to clear terrain.

The NTSB listed the probable cause as "the pilot's improper decision to fly directly up the center of the valley and not to circle to gain sufficient terrain clearance altitude...[and] improper route performance planning by the pilot."

But this article isn't about flying in the mountains. It isn't about blame, either. It's about what we might be able to learn and how we can try to prevent performance-related accidents from happening to us.

It is not known whether the pilot had computed climb performance before takeoff or whether some unknown factor played a part in the aircraft's failure to clear Independence Pass. One thing we do know is that climb data for the 172R is based on flaps fully retracted. At the instant of impact, wing flaps were deployed 20 degrees.

What we can learn from performance charts

The Aspen crash and other performance-related accidents can help us think through, understand, and avoid the mistakes of other pilots.

There are at least four lessons every pilot can learn from the Aspen accident:

• Pilots need to acquire a preflight "feel" for the performance of their aircraft under current and forecast circumstances--and then make the time to check out marginal situations.
• Marginal en-route conditions (like rate of climb, in the Aspen accident) need to be constantly assessed in flight, and immediate corrective or preventive action must be taken in the event abnormal, unexpected, or adverse conditions occur.
• As a general practice, information obtained directly from performance charts should be used to assess aircraft capability on every flight before every flight.
• Regular review of performance charts and related performance data can improve overall awareness in all aspects of flying.

Most pilots are generally familiar with the performance of the aircraft they fly, but too few of us actually compute "performance" from the charts, except on cross-country flights. Even then, how many routinely compute climb performance--which proved to be a critical factor in the Aspen mishap? Or takeoff distance--except when conditions seem unusual, the runway is a little shorter than what we're used to, or the aircraft is at or near the maximum gross weight? Or landing distance?

Having admitted that, what else might most of us not compute from the charts that we probably should?

Noteworthy performance notes

As the old saying goes, "The devil is in the details." In performance charts, those details appear among the notes printed on the charts themselves.

It pays to fly the airplane by the book, too. I always figured that someone smarter than I am (like the manufacturer) designed and built it, and that the test pilots burned a lot of fuel to test it and record how the airplane needs to be flown. We need to pay attention and do what the book says.

Though nobody knows how long the Aspen pilot's flaps had been set at 20 degrees, climb performance in the pilot's operating handbook (POH) for the Cessna 172R is based on having the flaps up. So is climb performance for the older Cessna 172N, which has a climb chart with that same note--flaps up--plus three other conditions and four additional notes, all of which specify other parameters pilots must follow to achieve the climb performance shown on that page. If they aren't met, the information on the chart is invalid.

In another example, there are two charts for computing short-field takeoff performance in the Cirrus SR20; seven separate conditions are specified on those charts for the computed data to be valid. In the POH, these two charts are accompanied by an additional page containing a sample performance problem and further explanations that should be followed.

Samples of "notes" in the fine print from performance sections of various general aviation POHs are listed below, grouped by the phase of flight to which they apply. Check out the flight manual or POH for the airplane you fly. Performance guidance varies considerably from manufacturer to manufacturer and from model to model. A review of 15 separate POHs revealed performance sections ranging from 13 to 32 pages.

Paraphrasing of manufacturers' performance charts, plus computational tips, notes from performance pages, FAA guidance, and other authoritative information are summarized below, by phase of flight.

General "performance"

• Load your airplane properly (both weight and balance); improper loading can seriously affect performance.
• Use the performance charts; don't extrapolate beyond them.
• Charts are corrected to "standard day conditions" and do not address varying degrees of aircraft mechanical deterioration or pilot proficiency.
• The effects of wind should be considered for climb, cruise, descent, and landing.
• Be conservative in all computations; don't "live on the edge."
• xisting and forecast conditions at departure, en-route, and destinations should be obtained before performance planning is begun; use actual landing weight for landing computations.
• Understand and use whichever temperature scale your book demands: Fahrenheit or Celsius.
• Make sure you're using the right speeds: KIAS or mph (especially if you fly more than one airplane, it can make a significant difference); calibrated versus indicated makes a difference as well.

Takeoff, ground roll, obstacle clearance

• Ensure proper tire inflation.
• Lean the engine properly on runup, climb, and cruise--there is a significant power impact.
• Use correct takeoff temperature (ATIS is not ambient; many manuals specify ambient); density altitude versus pressure altitude for various manuals.
• Prescribed flap settings for normal and short-field landings vary, even within brands; check the charts.
• Use the correct performance chart (i.e., short field versus normal takeoff).
• Note specific obstruction, liftoff, and rotation speeds (some are specified).
• Investigate runway slope and the general airfield environment (see "Accident Analysis: It's What You Know," October 2005 AOPA Flight Training).
• Disregard takeoff headwind in computations--what if you count on it and it stops? Do not take off in tailwind; depending on your liftoff speed, departure in a 10-knot tailwind could increase your takeoff distance by 21 to 56 percent.
• Observe performance chart limits; some charts stop at 8,000 feet. Don't extrapolate.
• Hold brakes, check full throttle...then release brakes.
• Most speeds are based on zero instrument error (but most instruments have errors; some are very significant in the area of stall speed).
• One manual notes that dry grass will increase ground run by 15 percent over book value.

Climb (rate, distance, time)

• Are wheel fairings attached or removed? Performance effects vary; use proper corrections.
• One general aviation manual says to increase climb time, fuel, and distance by 10 percent for every 10 degrees Celsius above standard temperature.
• Corrections for cruise altitude and departure altitude temperatures (called temperature departure) should be applied to climb computations.
• Proper gross weight.

Cruise

• Compute and apply cruise-altitude temperature deviation from standard temperature.
• Note specific cruise parameters: 55-percent power, 65 percent, 75 percent, etc., and specific performance chart notes.
• Note limits on charts and don't extrapolate.
• Check correct settings for best power, best cruise, economy cruise, best endurance, prescribed leaning, EGT (peak or 50 degrees from peak, etc.).
• One handbook says, "Reduce range 7 percent if wheel fairings not installed."
• Another handbook: outside air temperature noted in Celsius, EGT noted in Fahrenheit; which is yours?
• One manual differentiates between "endurance best economy" and "endurance best power."

Landing performances

Many of these points are in documents other than the performance charts.

• " Options are important; read the fine print on every chart--one manual specifies data valid only for "standard wheels, tires, and brakes. Power off, 40 degrees flaps, paved level dry runway, full-stall touchdown, max braking." It also has separate data for "heavy duty landing gear." What does your POH say?
  Flight manuals and handbooks seldom mention grass, soft runways, hydroplaning, and other factors that have effect, though some comments are listed below; they won't be found in most manufacturer materials, though one manufacturer says, "add 15 percent for dry grass." Some comments from the FAA's On Landing pamphlets are:
• If the runway is wet, airplane braking might be ineffective because of hydroplaning--sliding on the water.
• "Book" landing data was derived from perfect technique by factory test pilots under ideal conditions. Most computations are based on touchdown at 1.1 times stall speed. If you touch down even 10 percent above that, your landing roll will be over 20 percent longer than "book."
• Flying "final approach" airspeed into the flare (rather than slowing to 1.3 times the stall speed as recommended in most manuals) nearly doubles your landing distance; if you're carrying extra airspeed for gusts (as you should), understand that it will cost you even more landing distance to dissipate it. Developing a "feel" for these book values can be priceless when faced with marginal situations.
• Handbooks generally use either a specific speed (usually 1.3 times stall speed) based on gross weight or a specific recommended speed on final approach. The small print might or might not specify other speeds. Check it out. Take care how you manage all of them.
• Many pilots use the "bottom of the white arc" to determine stall speed when, in actuality, that is stall speed with flaps down at maximum gross weight--if your flaps aren't fully down or the aircraft is loaded to less than max gross weight, this will not be your stall speed.
• Even 200 to 300 pounds in a general aviation airplane can make a difference in your "float" and cost you precious runway space.
• Landing in a 10-kt tailwind--and touching down 10 kt fast--will almost double your landing distance; do you have enough runway? Don't land downwind unless you know you can afford it.

In the case of the Aspen accident, pausing to do more detailed performance analysis or climbing to altitude before proceeding on course probably would have prevented the accident. But it all would have had to start with awareness that there might be a problem because of the environment or the nonstandard temperature--awareness and concern over aircraft climb performance, or the lack of it. Perhaps the pilot was aware, but the press of time could have been perceived as a bigger problem. Whether it's mountains, rising terrain or trees off the end of the runway, temperature, or weather--or whatever else--the message is still the same: When in doubt, check it out.

Wally Miller is president of an aviation training, consulting, and marketing firm in Monument, Colorado. He is a Gold Seal CFI who has been instructing for more than 30 years and flying for more than 40.

Want to know more?

Links to additional resources about the topics discussed in this article are available at AOPA Flight Training Online (http://ft.aopa.org/links).