A strange thing happened after a four-hour IFR flight in the soup the other day. The strange part was not the visual approach or the six-mile visibility and the 6000 feet of runway ahead, but rather the energy momentum. I’ll add a bit of physics here. I was contemplating a nice landing with three souls on board and bags. In that contemplative mood, I turned a one-mile final for traffic considerations as per the tower at 800 feet. No biggie, one might say, and it shouldn’t have been. By the time Garmin’s faithful voice stated “500,” I was already looking at four white lights on the PAPI. And by the time I heard “minimums, minimums” (I program that on every flight where an approach is available), I was rushing in at 85 knots or 9% over the VSo1.3.
Arriving at the runway with too much energy will lead to a long flare.
Now in all likelihood most of us have done that one time or another. Maybe? And I have too, plenty of times, but determinately? Almost, never! Yet here I was plowing the slight crosswind hankering to take me off the runway. Nope, that was not going to happen. Even with a forward slip, I leveled off around 79 knots and then held and held and held till the lift dissipated and I touched down 750-800 feet past the threshold and ran the wheels for another 1400-1500 feet and then on to the third taxiway (and not a high speed one either). “Hmm,” I thought, “hope no one was watching.”
One might say, so what?
Let us look at energy as a stored form of force. As in potential energy that one gains by appropriating first the chemical energy (burning of fuel) and translating it to the mechanical energy (turning the prop and creating thrust) into stored potential energy as altitude.
You go up in the air with a whole bunch of fuel burn and then coast down with a bunch less. But in that bunch less is a major wizardry of airmanship. How we manage that energy is what determines the difference between the sound generated by the repeating Doppler-effect-engine-power-hog-jock and an aviator.
The slow dissipation of energy by carefully manipulating the throttle to achieve a steady state loss of that potential energy transformed into kinetic energy, is the key to good airmanship. When the landing configuration is best at 1.3 VSo for normal landings and 1.2 VSo for short field landings, there is a specific need to adhere to that tenet. Or else you run the gauntlet of what might be. And most times it won’t be pretty.
Many a pilot has ventured past the runway end carrying more energy than needed. A case in point is the recent Falcon crash at Greenville Airport that killed both the pilot and copilot in their haste to land while the runway kept shortening in front of them. The NTSB has yet to finalize on that fatal accident, therefore, this statement is a hearsay opinion based on the video of that flight while in the landing phase.
Remembering that speed is a form of kinetic energy and it dissipates at a certain rate when thrust is eliminated: the stored energy is a thief of time and space. Knowing only through practice, conditions a pilot in its judicious use.
Let’s look at the correlations between speed and energy…
Assuming a 3000-pound aircraft arriving at a runway at the given speeds creates the resultant force. (Kinetic Energy + ½ mass * Velocity squared):
- 65 knots of airspeed = 562,128 ft-lb of kinetic energy. 0% Baseline
- 80 knots of airspeed = 851,508 ft-lb of kinetic energy. +52%
- 50 knots of airspeed = 332,620 ft-lb of kinetic energy. -41%
Many runway overrun accidents are the result of too much speed or altitude – energy.
Given those dynamics, it behooves us to maintain the appropriate energy on final approach to the runway.
Additional benefits of proper speed/energy management also include:
- The sudden deceleration, say, hitting a parked truck or a deer on the runway or a structure requires tremendous energy dissipation.
- Stopping a 60kt aircraft at 18 feet distance leads to a 9G force on the body (the limits of the FAA certified seat belts).
- Stopping that same force/energy in 9 feet distance leads to an 18G force on the body (limits of sustainability).
- And a sudden stop at 1-foot distance leads to 159 G force (unsustainable for the human body).
So, going back to my landing, I came in with near 10% higher speed/energy than required and I paid the price in runway used. Mind you, with that much stored/kinetic energy, I would certainly have taken out the runway end identifier lights on a 2000-foot runway.
A word of advice: translate appropriate power and pitch for climb/cruise, appropriate power reduction and constant pitch for descent and zero power and changing pitch for level over runway and thence a smooth landing flare. Always concentrate on the VSo1.3 on normal landings and VSo1.2 on short field landings.
Fly by the numbers appropriate for the aircraft you fly and always fly safe.
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I remember landing solo at Wings Field in a Tiger after flying from Toronto and stopping for customs at Philly International. I had too much float after an approach at book airspeed. The second approach got the wheels down on the hump well down the runway. I got the lesson that book approach airspeed is correct at gross weight. The adjustment factor is the square root of (landing weight / gross weight).
Now for some more aerodynamics:
Once on the runway, the speed used to determine kinetic energy is the groundspeed.
A 65 kt true airspeed into a 10 kt headwind yields a kinetic energy speed factor of 55 squared = 3025
With a 10 kt tailwind, the speed factor is 75 squared = 5625
making 86% more work for your brakes less a small contribution from drag and rolling resistance countered by residual thrust.
…”book approach airspeed is correct at gross weight. The adjustment factor is the square root of (landing weight / gross weight).”
Thanks for this. I’d never been taught this before. In the 152s and 172s I trained in, it probably was an insignificant change between gross and a “usual” landing weight. However, my current plane can have an 842lb swing between one pilot with one hour of fuel and full-up gross weight. That’s an 11kt difference in Vref. No wonder I floated on landing when light!
After reading this, I did two landings with a 250# fuel difference. Both landings, at their computed Vrefs, felt the same. Consistency is key to good landings—you’ve added a component of consistency to mine.
Great story, now if anyone can tell me how to land a 182 in a decent fashion with 2 guys in the front seat, I am all ears. 40 years of flying, many types, but the Skylane and I just don’t get along over the numbers.
A couple of cases of oil in the baggage area will help immensely.
Surprised that nobody suggested that to you when you first trained in the airplane.
Yes foreword CG does present an opportunity to set up wrong.
Besides adding weight to baggage note it helps top speed too; go can setup a higher angle of attack by putting the manifold pressure at 17. This will give you a nose up decent attack angle instead of using nose down from zero power, it allows you to relax back pressure to increase IAS rather than going over center with nose down. In my 210 if I come in power off and get below that angle of attack with the forward CG I run out of elevator at rotation. The weight in back makes this much less acute but a high angle of attack is a bit steeper on decent with no additional air speed, from there the elevator is still effective. Took me a time to realize it was not me but a characteristic of the plane that was designed to carry load behind the forward two seats.
Oil in baggage makes a wider envelope or more margin of error for the rotation.
Higher angle of attack seems to keep the elevator out of the wind wash and increases leverage.
It took a friend telling me to add slight power to tame the beast. It worked great and over time I learned to set it up with power and backed off the power until I could descend power off with the same result.
I’m an armature but this was my experience and my results are much improved
Follow the soft-field landing technique recommended in the Airplane Flying Handbook pg 8-21.
Reading the NTSB report on the Falcon crash, it doesn’t sound like they landed with
too much speed. More like the engines refused to throttle back – 2 fire handles were
pulled. Very peculiar – can’t wait to read the final NTSB report.
Got to agree with you. While it doesn’t diminish the premise of the article, it’s a poor example to use since the NTSB preliminary clearly states the engines ran at full power for 20 minutes after the first responders arrived.
Great article but for one error: “So, going back to my landing, I came in with near 10% higher speed/energy than required and I paid the price in runway used.” It is either 10% higher speed, *or* 10% higher energy. Kinetic energy is proportional to speed-squared. The author clearly knows this (per the rest of the article) but that sentence is in error.
Interesting Parvez. I always enjoy a scientific approach to approaches. Although not a learned physics expert, I have spent my adult life thinking of these things and learning by field trial and not as many errors as close calls.
Your article got me thinking of why I occasionally find myself too high and too fast. Usually this happens when returning from a cross country flight.
After some reflection here, I’ve summed that in my 20+ years of flying, almost all of that time has been in aircraft that came from my own wallet. My concern of shock cooling has always led me to squimishly pulling the power back at (what I was told once by a mechanic friend) 1″ a minute for a safe rate of cooling. I never seem to start this ritual soon enough coming in from altitude, leaving me too fast with little distance to disapate.
I fly a 185 and again this only happens when I have this aircraft in cross country mode. When the aircraft and I are switched into back country mode, I’m always over the intended touchdown spot at closer to Vso than the book would suggest. With a little headwind, it feels like you could jump out at touchdown and jog beside the aircraft.
Thanks for the reminder to be patient and besides, why would I ever be in a hurry to get back on the ground?!
There are 3 rules to making a good landing. The problem is no one knows what they are.
Dana, I agree with you. In aviation, we say we use mathematics in finding answers to operating airplanes. That may be somewhat true, but the problem is, we measure with micrometers, mark with crayons, and cut with axes. It’s all in the feel, not in the numbers. And I can tell you this: It’s exactly the same in a A320 or a B767 as it is in a 172. I’ve been doing both for years, and there ain’t no difference.
David. The feel comes after the numbers. They are both important, otherwise why would you need a copilot in your Boeing to make sure you’re flying at Vref +5?
It can be done without the feel part (many copilots haven’t got it yet) but it is seldom pretty or elegant.
You order a suit by the numbers. Then, after it arrives, you take it to a tailor and have it altered until it feels right. Nothing that takes human touch to manipulate can be completed without ‘feel’. When you get to the point where you too can judge from both seats, Cessna and Boeing, you’ll be able to call it accurately.
You have to interpret what your instruments are telling you, not the seat of your pants. Just like being in a car on a long trip with high steeds for a long time, you slow down at a small town and soon you find yourself speeding because you are used to being at a high rate of speed, it takes a while to get reused to the slower speed. But in an airplane you must believe what the instruments are showing and nothing more, trying to land to fast is the pilots fault exclusively and nothing else, anything else is nothing but a self made mistake trying to make one feel better of messing the landing up, nothing more..
Great article. As a former glider pilot it is all about converting potential energy into kinetic energy and back again. Filling the wings of your glider with water ballast really demonstrates this. A powered aircraft may have plenty of (externally provided) energy in the form of fuel, but the mass of the aircraft is a huge source of potential energy – ready and willing to become kinetic.
The winds aloft at my local airport are often much different than the Awos says. I generally have no trouble landing elsewhere, but often find I have a tailwind at final until a hundred or so feet from the ground at my local airport. This situation makes energy management a nightmare and often leads me to look at my ground speed on my GPS for reference. Seems to be a more pronounced problem in the winter months at this locale. This may be the case at different airports around the country. Local knowledge is a great thing, but an occasional glance at your GPS ground speed isn’t a bad thing in any case.
In case any of you want to follow the math, measure everything in the pound-foot-second system.
Convert V speed to ft/sec: Knots x 6072 / 3600 = ft/sec.
Convert weight to mass: lbs / 32.174 = slug mass.
Kenetic Energy KE = 1/2 x slugs x ft/sec squared
So 3000 lbs / 32.174 = 93.24 slugs. 50 knots x 6072 / 3600 = 84.33 ft/sec. KE = 1/2 x 93.24 x (84.33 x 84.33) = 331,540 ft-pounds
I worked this up for the listed stall speeds at gross weight vs pilot only weight for my Mooney E. It is very enlightening.
Thank you for writing this article.
Just a quick addition to my comment:
For a Mooney M20C or E the gross weight is 2575 lbs but with pilot only and about 11 gallons of fuel it weighs 1900 lbs. The book stall speeds at GW MPH are 67 no flaps, 64 TO flaps, and 57 with full flaps. Knots are 58.26, 55.65, and 49.565 respectively. Converting to Feet/Second the speeds are 98.27, 93.86, 83.60.
Correcting stall speeds for 1900 lbs then the correction factor is (1900 / 2575) SQRoot = 0.85899, or about 15% slower. Multiplied out the new stall speeds for pilot only are 57.55, 54.97, 48.96 MPH. That is about 10-9 MPH slower at light weight. Knots are 50.04, 47.80, 42.58, or about 8-7 knots slower. Ft/sec are 84.4, 80.625, 71.81.
The Kenetic Energy at the corrected speeds compares as follows:
KE at 2575 GW ft-lbs: 386,441; 352,535; 279,676
KE at 1900 GW ft-lbs: 210,331; 191,937; 182,261
That is about 35% less energy at the low weight with the 15% slower stall speed, or about 50% more energy at the higher stall speed compared to the lower stall speed.
It is enlightening to see the numbers in front of you.