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Forgotten Factors in Performance

forgotten factors in performance Airplane GEEK Forgotten Factors in Performance

By Robert N. Rossier, EAA 472091

This piece originally ran in Robert’s Stick and Rudder column in the July 2021 issue of EAA Sport Aviation magazine.

One lesson many of us have learned in recent months is the price we pay for complacency. It isn’t that we don’t know the rules that apply, or how to apply them, but we simply become lax in following them. In aviation, that can be a serious mistake.

One of the conditions pilots need to avoid is falling into patterns that can lead to complacency. It can happen when we fly the same aircraft, loaded the same way, to and from the same airports, and under the same or similar conditions. There is a comfort that comes with repetition. We know what to expect in terms of performance, and the airplane performs as expected. But, at some point, some of the critical performance factors that we haven’t recently considered begin to fade from disuse. Worse yet, we might simply forget those factors entirely when a change to our normal operating environment does occur.

Runway Conditions

One such scenario occurs when we venture to an unfamiliar airport and runway. What we might forget to consider is the effect that different runway conditions can have on performance — both for takeoff and landing.

Runway slope — even a seemingly gentle slope — can have a marked impact on takeoff and landing distances. In most circumstances, we want to land upslope and take off downslope. Even a 1-degree upslope causes takeoff ground roll to increase by 22 percent, whereas a 1-degree downslope for takeoff is like having an extra 10 percent of runway length. That’s just 1 percent — a 25-foot change in elevation over a runway length of about a half-mile. Taking off with a 2.5-degree upslope could be like taking off with a 7-knot tailwind.

Another factor we can easily overlook is the runway type. The performance data we find in our pilot’s operating handbook (POH) is typically based on dry pavement, but performance can vary considerably when we venture onto turf or grass runways. Without any formal guidance to help us out, the best we have might be some rules of thumb that can help us estimate the effects. On a firm turf runway, we can expect our ground roll to be 7 percent longer for a takeoff. If the runway is short grass, we need to add 10 percent to our ground roll for a takeoff. For tall grass, we might need to add 25 percent to our takeoff roll.

But here is the kicker: Although we might expect a shorter ground roll with turf and grass runways, the ground roll and stopping distance can be much longer if that grass is wet due to the low friction factor!

Weight

It’s pretty clear to any trained pilot that weight is a critical factor in aircraft performance. Our POHs commonly provide us with good data regarding performance as a function of aircraft weight. Students on their first solo flight are usually surprised to find their trainer rocketing skyward without the excess weight of that instructor onboard. The opposite applies when we add a couple of adults in the back seats of our four-seat airplane. The climb performance becomes much more sluggish than it was with only one or two occupants onboard.

We need to keep in mind that both stall speed and maneuvering speed are published for a specific weight and are impacted by changes in aircraft weight. Here again, we have a rule of thumb to estimate the effects. For stall speed, the percentage increase in stall speed is about half the percentage increase in weight. For example, if we increase weight by 10 percent, our stall speed goes up by about 5 percent.

The same applies to maneuvering speed. We know that as our weight decreases below max gross weight, our maneuvering speed decreases as well. The decrease in maneuvering speed is half the percentage of the decrease in weight. That’s something we might need to be aware of as we encounter turbulence or a traffic pattern when lightly loaded.

Density Altitude

Especially in the heat of summer, another factor we need to consider quite seriously is density altitude. Our POH typically includes performance data for other than standard conditions. Consider that a normally aspirated engine experiences a roughly 3 percent reduction in power for every 1,000 feet of density altitude, and density altitude can be much higher than field elevation.

Density altitude can significantly impact takeoff and landing performance. As density altitude increases, so does the true airspeed required for flight. As a rule of thumb, true airspeed increases by roughly 2 percent for every thousand feet of density altitude. So, at a density altitude of 5,000 feet, the true airspeed will be 10 percent higher than indicated. That means the aircraft on takeoff must accelerate to a higher groundspeed to achieve flight. Likewise, for a landing, an aircraft will be traveling at a higher groundspeed when approaching for landing when density altitude is increased. Since the aircraft touches down at a higher groundspeed, it takes more runway distance to decelerate and stop.

Density altitude also impacts an aircraft’s climb performance. The effects might seem minor, but they do add up. As pilots, we memorize such numbers as best rate and angle of climb airspeeds, but these are not constants. Some simple rules of thumb can help us estimate the effects. For every 1,000 feet of density altitude, VY decreases roughly 1 percent, whereas VX increases about 1 percent. If we’re trying to squeeze out the maximum performance from our airplane, we need to consider these factors.

We should also keep in mind that climb angles are increasingly shallow as density altitude increases due primarily to several factors, including the higher groundspeed at which it is flying, reduced propeller efficiency, and the reduced power available.

While many airports have either an automated surface observing system, automated weather observing system, or automatic terminal information service that provides density altitude, such isn’t always the case, so it helps if we know how to estimate the density altitude. One way to do that is to remember that density altitude increases about 600 feet for each 10 degrees Fahrenheit above standard temperature for field elevation.

Technique

Yet another factor to consider is our level of skill in properly applying flying techniques. For those who practice often, the payoff comes when those skills are called into action. But if we haven’t been polishing those skills, we can expect some degradation in our performance, which likely translates to lower aircraft performance.

A Confluence of Factors

A situation can quickly become serious when more than one factor is affecting our performance. Add a sloping or grass runway to a heavily loaded aircraft or a high-density altitude scenario, and a familiar aircraft can seem alien in regard to takeoff and climb performance. Again, we can use a rule of thumb to help keep us in the safety zone. An aircraft should reach 70 percent of takeoff speed at the halfway point of the runway. If not, it’s time to abort.

It’s easy to become complacent and forget some of the factors that affect performance. By focusing on those sometimes-forgotten performance factors, we can help tip the scales of safety in our favor.

Robert N. Rossier, EAA 472091, has been flying for more than 30 years and has worked as a flight instructor, commercial pilot, chief pilot, and FAA flight check airman.


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