Aircraft performance is black and white
Whether you're a bush pilot or a weekend flier, it shouldn't take a ramp check to teach you the importance of having a pilot operating handbook (POH) on board. Even if you never get busted, it goes beyond being legal. These guides provide important numbers — the envelope your aircraft operates within — that is if you care to open them.
But where do these numbers come from and what do they mean to you as a pilot who wants to polish your flying skills? First, you need to have an appreciation for why they exist. For most of general aviation's history they simply weren't required. Way back when, they were called "owner's manuals" and offered at least more detail than what you might find for your Black & Decker vegetable steamer or AMC Gremlin. Other information was passed along by what is referred to in the industry as "tribal knowledge," meaning that which is handed down from other pilots but may not be what the manufacturer recommended.
A tattered old owner's manual for the Piper J3C-65 Cub shows a couple sketched on the cover happily flying along. All the relevant information is scattered about the passport-size booklet, but when you get to the end there's the helpful "Ten Commandments for Safe Flying." These include: "Thou shalt maintain thy speed lest the Earth arise and smite thee; thou shalt not let thy confidence exceed thy ability; and thou shalt not become airborne without checking thy fuel supply."
The same safety information is communicated in modern POHs, except it's in a much more stilted manner. You also may have noticed some similarities between POHs for different aircraft, and this isn't simply a matter of copying or coincidence. March 1, 1979, marked a key date in the semitumultuous history of aviation technical writing. From that day on, all aircraft were required to have approved airplane flight manuals (AFMs). These are the most complete sources of information because they are updated by mechanics or owners with manufacturer data. POHs were already around by the time the AFM requirement was enacted. They came on the scene in the mid-1970s as a result of a partnership among the FAA, industry groups, and companies to standardize and expand information in owner's manuals. The modern form is referred to as the General Aviation Manufacturers Association (GAMA) format. A POH can substitute for an AFM as long as the flight manual is a component part of the POH.
Then there are information manuals that provide good information but are not specific to a particular aircraft by serial number. You can pick these up at the local FBO before you rent aircraft. Cirrus Design Corporation uses technology to take things a step further. In addition to a POH, performance data and checklists are available on the multifunction screen in SR20 and SR22 aircraft.
Not all manufacturers have adopted GAMA's format. Take a look at the POH for American Champion's Super Decathlon. Jerry Mehlhaff Jr. of American Champion said that the company based it on the old and much more detailed Bellanca format used for Citabrias before Bellanca ceased production. One thing that stands out is the Aresti figures for the aerobatic maneuvers that the aircraft is capable of. As an airplane that is mostly flown by feel, it does have its fun side.
The language in POHs is not exactly frozen. Based on input from flight schools and owners, the manufacturers continue to tweak the manuals. Dale Bleakney, senior Cessna engineering flight test pilot, describes the GAMA format as a "revolution" while the updates to the handbooks are part of an "evolutionary process." Beginning in 1996, with the restart of single-engine production, Cessna rearranged the order of the pilot checklist items to make them more user-friendly.
The handbooks are organized by the normal sequence of flight. The Cessna POH features abbreviated as well as expanded normal and emergency procedures. In addition, performance tables or graphs are included for specific flight operations. With the new generation of airplanes, performance information is now provided in the abbreviated checklists.
One of the biggest problems Cessna sees is owners — who were perhaps brought up on carburetion — failing to familiarize themselves with or receive proper instruction on newer features such as fuel injection. A lot of this stems from simply not reading the POH, Bleakney said.
It should be pointed out that the numbers in the POH represent the performance of a perfect pilot and flawless airplane. The test pilot will attempt to use "average pilot skill," but the performance figures are obtained by using a predetermined set of conditions and pilot techniques. More than a decade ago the FAA issued an advisory circular that provides guidance for flight-testing airplanes. Under the framework, a pilot from Cessna should be able to hop in an airplane produced by Raytheon or Piper and get the same data as that specified in the respective POHs. Cessna uses a test airplane that is outfitted with elaborate instrumentation. A pitot probe is used instead of a pitot tube to reduce pilot error. Before flight, the test team checks everything related to the test such as engine horsepower, propeller rpm, manifold pressure, etc. The flights are done on calm days, something verified from using wind stations that can give, say, the winds at 30 feet. The weather information is much more accurate than what a control tower might provide. After the data are gathered, the numbers are calculated using engineering formulas. If the data appear inaccurate, more flights will be conducted. After certification, there are production flight tests. In sum, the numbers represent something you will only achieve using the same techniques on a similar day, with all the variables known. Bleakney recommends using the numbers as a baseline while building in fudge factors derived from your own experience.
A good way to learn just how much you should fudge is to use the "rules of thumb" in The Axioms of Flight by James Embree. In the book the author uses a number of calculations that cover all realms of flight. In the takeoff performance chapter, Embree writes: "A 10-percent change in aircraft gross weight causes a 21-percent change in takeoff distance. Do not use this rule if weight change is in excess of 50 percent." To maintain the best-rate-of-climb speed, Embree recommends reducing the sea-level indicated airspeed by 1 percent per thousand feet. In the cruise performance chapter, Embree says that to maintain the maximum-range indicated airspeed, reduce airspeed by 10 percent for each 20-percent decrease in weight. And in the landing performance chapter, there are many useful tips such as: "Crossing the threshold 50 feet too high increases the landing distance by 25 percent."
The people who probably know the most about fudge factors are ferry pilots. Globe Aero Ltd. of Lakeland, Florida, simply can't take the numbers in POHs literally. Because the company ferries aircraft throughout the world, it flies 25 to 30 percent over gross weight to accommodate the needed extra fuel tanks and equipment under ferry permits. The extra weight changes all the POH data. The one thing that does remain the same is how to start the engine, quips Phil Waldman, Globe Aero president. The company uses the POH as a reference combined with the pilot's own experience with similar aircraft. Waldman said that because most aircraft haven't changed much over the years, the manufacturers have perfected the data and he finds POHs to be highly reliable.
But what if you're in a jungle and all the information you have on the aircraft is on eight typed pages? As a longtime missionary pilot for JAARS (formerly called Jungle Aviation and Radio Services), a training and coordinating organization for technical and logistic support services for Bible translation work based in Waxhaw, North Carolina, Bob Griffen has faced just that. In 1955 Griffen took delivery of Helio Courier serial number 22 and quickly concluded that few people knew anything about getting the most performance out of these aircraft, including the company. "The learning curve was very steep, but it's a great airplane," said Griffen, now retired after more than 7,000 hours of flying in South America and the Philippines.
While the missionary pilots proceeded cautiously and discovered the incredible short-field performance the Helios were capable of, others took a more scientific approach. In the early 1970s, Don Smith used the basic performance data from the factory and expanded on it. He then programmed a Hewlett-Packard scientific calculator so that pilots could enter the flight conditions such as winds and humidity to determine if they could take off out of the bush. The calculator was also used for making weight and balance and navigation computations. (Today there are many mechanical and electronic flight computers on the market for calculating aircraft performance.) The approach proved remarkably accurate in the field, and it was further vindicated when Smith went over accident records at JAARS.
By plugging the conditions into the calculator, he realized that he could have prevented the crashes that he reviewed. One involved a Helio in Liberia where the pilot was confronted with an unenviable situation: take off uphill into the wind or downhill with the wind. The pilot chose wrong for the particular conditions and took off uphill, trying to outclimb the terrain. Smith said he could have predicted the point of impact.
The calculator later saved Smith when he was with a trainee on a flight in the mountains of North Carolina, where he was able to build in a safety factor that overcame the student's mistake on takeoff. The data have since been turned into performance tables so that missionary pilots don't have to do the calculations. JAARS has, in effect, created its own POH for the Helio. And the group's safety record speaks for itself. Thanks to good training and maintenance, and understanding performance, JAARS has maintained an accident rate of 1.7 incidents from nearly 20,000 flights a year in some of the most inhospitable places on Earth.
E-mail the author at [email protected].
Putting It Into Practice
Making book
A perfect pilot and a flawless airplane? Though the 1998 Cessna 172 I flew for this article was in reasonably good condition, it could hardly be called flawless. As for my own flying skill, whenever I feel things are clicking along smoothly, that's when I know to start watching out — because the universe has a way of sending "perfect" pilots a timely comeuppance.
Searching for some rules of thumb from the 172's pilot operating handbook (POH) certainly wouldn't take a perfect pilot nor a perfect airplane — in fact, average was all we needed to be, the airplane and I.
I chose to look at some basics, the procedures that returned the most in safety and usefulness, so I focused on takeoff and landing, climb performance, and fuel burn. Runway 23 at the Frederick Municipal Airport in Maryland is 5,220 feet long, lined with lights that I would use for measurement during the takeoff and landing tests — tests that pilots could easily replicate with their own airplanes at any airport. Cessna rounds the numbers in its POHs to the nearest 5 feet; I would be happy if mine came to the nearest 50 feet, rounding up to stay conservative. I wasn't out to prove anything — only to find out how the combination of my skills and the aircraft's performance would team up on the playing field.
Another pilot flew with me to take notes, as I would concentrate on flying as close to the recommended airspeeds as possible, using techniques outlined in the POH. We started with full tanks, putting us about 210 pounds below max gross weight. An ASOS-reported headwind that gusted between 8 and 16 knots also helped the airplane's performance, but it challenged me to keep the airplane as smooth and stable as possible.
In two separate short-field takeoffs, the results were about the same. I lined up at the very end of the runway and followed the POH's short-field takeoff checklist, which advised holding the brakes as I advanced the throttle, and releasing them after I confirmed that engine instruments were in the green. The expanded procedures in the POH noted that this technique should not be used on gravel runways, as the standing runup picks up rocks — a good example of the tips hidden in that section.
On the first attempt, the ground roll lasted until we came to the first runway light past the first taxiway — about 1,000 feet. We cleared the proverbial 50-foot obstacle by the time we reached halfway between the second and third taxiways, about 1,800 feet from the threshold. Cessna's 172R POH notes distances of 980 feet and 1,745 feet, respectively, for a sea-level takeoff. Taking into account the lofty altitude (303 feet msl) at Frederick, the 172 performed to the book on the 21-degree-C (nearly standard) day. However, it's worth noting that few pilots adhere strictly to rotation and best-angle and rate-of-climb speeds. But if you can, take heart in these POH numbers.
Landings told a similar tale: Using full (30 degrees) flaps as dictated by the manual, I aimed at the numbers (simulating that pesky tree placed just short of the threshold), touching down before the first taxiway, and rolling to an easy stop by the first light before the second taxiway. This equates to about a 1,600-foot landing over the obstacle, and a leisurely 700-foot ground roll, versus 1,310 and 560 feet, respectively, from the POH. Adding 20 percent to the book figures and following the procedures to the letter are rules of thumb to follow.
After the short-field takeoffs and landings, it was time to climb. Since Frederick sits downwind of the last significant ridge of the Appalachians, with winds aloft out of the west I expected turbulence from 1,000 to 3,000 feet agl, and possible wave activity. Again, I knew I could not get ironclad numbers from a single flight, but instead a snapshot of how one might expect a slightly less-than-new airplane to perform as compared to book. As far as airspeed, I followed the rule of thumb to drop about a knot every 1,500 feet. The time to climb from 700 to 4,500 feet msl was 6 minutes, 40 seconds, for an average climb rate of 571 fpm. Book figures for such a climb offer a rate of 612 fpm; 6 minutes, 18 seconds of elapsed time; and 1.5 gallons of fuel used.
Without an on-board fuel computer, it was hard to measure the fuel burned during the time-to-climb test, though we used nine gallons during the 42-minute flight. Using the POH to estimate the fuel burn, I came up with seven gallons for a similar flight. While I used full throttle for the tests, I also made an additional full-stop landing with a taxi-back during the flight. The resulting fuel burn was quite high — extrapolating to 12.8 gallons per hour — even though I leaned aggressively during the climb. Though derived from a lone flight, this increased fuel burn is worth remembering, as I have seen fuel burns that are a gallon or more per hour higher than book in other rental aircraft. Whether a tired engine is approaching overhaul or improper use by other renters is to blame, it pays to stay conservative when estimating required fuel, especially when flying an airplane that isn't your own.
In fact, that's a good rule of thumb to follow any time. — Julie K. Boatman
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