RELATIONSHIPS ….

 Acknowledgements: Thomas P. Turner/ Mastery Flight Training, Inc. & Flight Instructor Hall of Fame inductee

“Have you ever noticed some of the relationships that apply to aircraft performance? Most pilots somewhat grudgingly learn to use the Performance Charts in the handbook for the airplane they fly …. and many pilots learn enough to get through the Written and Practical Tests, then stop paying much attention to the charts at all. 

 Today’s flight planning software helps immeasurably, but even so many pilots rely on approximations and rules of thumb …. usually these do work, but sometimes they don’t tell the whole picture. I encourage you to make a few calculations for each airplane type you fly; firstly to refresh yourself on how to read the charts, but secondly to identify some of the relationships between different performance figures, to help you more completely manage the risks and take advantage of the opportunities presented by the aircraft.

 Relationship 1: Take-Off Ground Roll Distance Vs. 50-Foot Obstacle Distance

·       Make three or four take-off distance calculations for an airplane you fly, at various temperatures and airplane weights at fairly low field elevations (a couple as sea level, a couple at 2000 feet, etc.). For most types, the Take-off Performance Chart gives two results: a ground roll distance (assumes zero obstacle) and a distance to clear a 50-foot tall obstacle. 

·       Compare the two results. You’ll probably find a rough rule of thumb for that particular airplane. For example, in the airplane I most commonly fly, a general rule is that the distance to reach a point 50 feet above ground level (assuming a maximum performance, VX departure) is approximately twice the ground roll distance. Put another way, if the airplane takes 1000 feet over pavement from power-up to wheels off the ground, it will take another 1000 feet (2000 feet from the power-up point) to clear a 50-foot tree or power line …. The relationship may be different for your airplane, but at least now you will know a bit more about the “big picture” if you are thinking about flying into a short runway with trees in the departure path. Considering flying into a 2300-foot grass runway with obstacles? Some pilots focus on the runway length alone. Knowing these relationships reminds you to check the obstacle clearance distance as well.

 Relationship 2: Take-off distances at low vs. high density altitudes

·       Using the same technique, make a few comparative calculations for take-off and obstacle clearance at the low altitudes you already checked, and then at a higher density altitude, say 6000 feet higher …. This helps you better visualise a winter vs. summer day, or that summertime trip to the mountains for a lower-elevation pilot. With a few comparison calculations, develop an understanding of how much your airplane’s performance changes with an increase in density altitude.

·       You may have already noticed on (your) Performance Chart …. that the slope of the lines is steeper at higher altitudes. This tells you that the relationship between ground roll and obstacle clearance distances is different at higher density altitudes.

 Relationship 3: Best Glide and descent profile vs. short field final approach rate of descent

·      Because of the popularity of tablet-based flight charts and moving maps, and the fairly recent introduction of a “glide ring” option that depicts the “footprint” the airplane can reach on the ground in an engine failure at Best Glide speed in the glide configuration, many pilots are becoming more aware of their airplane’s glide ratio. The glide ratio is the relationship between the amount of forward distance the airplane can travel for some amount of altitude lost in the glide, usually 1000 feet. 

·      Most light airplanes will have a glide ratio in the 8:1 to perhaps 12:1 range, with the actual amount varying widely between airplane designs. If this information is published for your airplane type, you’ll usually find it with the Glide checklist in the Emergency Procedures section, or perhaps in Limitations or Systems Description. 

·      Two other (glide) relationships exist in many aircraft:

1.    The Best Glide speed is very close to the speed recommended for final approach on the Landing Performance Chart, at least for airplanes with light wing loading - the Landing Performance Chart in most handbooks describes a short field technique

2.    In most airplanes I’ve flown you can visualise the glide radius in flight by drawing an imaginary circle from wingtip to wingtip as seen from the pilot’s seat. Anything on the ground within that circle is likely to be within your Best Glide range unless there is a strong wind. 

n.b: Consult your handbook and practice in flight to find the relationships that apply to the aircraft you fly.

 Relationship 4: Cruise speed vs fuel burn (knots per gallon)

Next time you’re taking a trip and you’re not in a huge hurry, try out a few different cruise power settings within the allowable range of any Aircraft Limitations. Repeat the exercises over time to compensate for the effect of winds on flight planning. The end goal is to look at the relationship between cruise speed and fuel burn, expressed in knots per gallon (litre for my international readers) as a way of selecting efficient cruise power settings, using whatever definition of “efficient” you choose for a given flight.

·       In an aircraft with a fixed-pitch propeller, establish cruise at a chosen RPM and mixture leaning technique. Note the cruise speed and fuel burn - if you don’t have a fuel flow gauge or fuel totaliser, you may have to do this on separate trips to determine relative fuel burn by noting how much fuel is needed to top off the tanks. Do your test runs at similar altitudes to remove variables from your observations.

·       In an aircraft with controllable pitch propellers, make a series of runs at constant manifold pressure or throttle position and different propeller speeds, or constant propeller speeds and varying manifold pressure.

·       Regardless of propeller type, you can do the same exercise at constant power settings with varying mixture management …. 

 Relationship 5: Cruise speed vs fuel burn (time and fuel to cover distance)

·       Take another look at the data you collected in the speed per fuel burn exercise above. Calculate the total time and total fuel burn to cover a set of distances, for example, 100 nautical miles, 250 nm, 500 nm and 1000 nm, at each of the selected power and mixture settings. Ignore for now the climb and descent phases. The idea is to look at the relationship between time and fuel to cover distances. 

·       You may find that aggressive leaning doesn’t save much fuel because of the time it takes on a longer trip, or that running the engine flat out doesn’t save much time but burns a lot more fuel on a shorter haul. You could even calculate the effect of tailwinds or headwinds on this “efficiency index”, and when it makes sense to fly slower but nonstop when flying faster requires a fuel stop. Again, this gives you some data to make decisions on whatever “efficiency” means to you.

 Relationship 6: 50 ft obstacle distance vs landing roll distance

·       Just as you did with the Take-off Distance Charts, do some comparative calculations with the Landing Distance Charts to find the relationships between the ground roll distance (0 obstacle) and the 50-foot obstacle clearance distance. Remember that this chart usually describes a short-field landing technique and maximum braking, with a steep-angle, power-off approach at a minimum safe speed. 

 In my experience, pilots may want to be more familiar with the performance of their aircraft, but unless they fly in a maximum-performance environment they rarely “crack the books” to see what their airplane can do. Sometimes this take an unfortunate turn when a pilot then tries to operate near the limits of their airplane’s envelope, then fails to outclimb obstacles after lifting off from a short airstrip, or runs out of fuel just short of their planned destination, or clears the power lines off the arrival end of the airport only to roll off the far end after touching down. Nothing replaces the need to calculate performance but knowing the performance relationships will make seeing “the big picture” and when you need to really get into the books and fly using maximum performance techniques, more obvious. Wouldn’t it make a great and informative Flight Review to spend your time together with a flight instructor …. Determining some of the performance relationships that apply to the airplane you fly?”

 FLY SAFE!