How smooth is too smooth? And how to achieve that?
Before we start the never-ending discussion about super butter/greased touchdowns, an essential disclaimer right from Boeing’s Flight Crew Training Manual: “A smooth touchdown is not the criterion for a safe landing.” Having said that, and emphasizing crystal clearly that a smooth touchdown is completely secondary and even not desirable in certain circumstances, we can now dive in—with a deliberately reduced sink rate—to this mesmerizing subject.
The first and most important thing: different airplanes land differently. And since we will eventually get to the widebodies, let’s summarize general aviation techniques first. No wonder taildraggers are harder to grease: the landing technique consists of stalling them right over the touchdown point. Yes, there are options, like landing the mains first and reducing speed and setting down the tailwheel gently, but those are not applicable to all operations for a myriad of reasons. Of all my landings, only one was in a taildragger, and although it was good enough for me to walk out of the airplane by myself and for the owner to use it again without extra maintenance, it is reasonable enough that I focus, from now on, on tricycle gear aircraft.
I learned to fly in a Cessna 152, a respectful trainer that is really generous and fun to fly. The flare and touchdown technique for a single piston Cessna doesn’t change a lot with its size. You are approaching with a rather high speed (we discussed months ago here in another article) and have to bleed this energy during the flare, touching down with the mains despite the fact that the design of the Cessna is already kind of nose up. So, if you are at pitch zero, you would touch with the nose gear first—never a good sight.
Hold it off… Hold it off…
Therefore, you are already at idle, well into the flare, and hold, hold, hold, until you touch down softly with the main gear—sometimes close enough to a stall to hear the horn. Most single engine pistons land the same, and although I haven’t tried myself, maybe even some single turboprops might land well that way (if you know, let us know in the comments below, please).
Remember: the airspeeds we use in a Cessna are all based on maximum takeoff weight, which is seldom going to be your weight during any landing due to the 172’s limited resources for air-refueling. I’m not getting into the crosswind landing techniques as well, since we extensively talked about them in another article a while ago.
On the ladder up to the heavy metal, you get on the next step: the light twins. I had the opportunity to fly two classics on this category, the Piper Seneca and the Beechcraft Baron. The Seneca, especially the first, lacks so much power and owns a so obvious out of trim condition that is not really practicable to flare it at idle; most of the time, you will close the levers as you touch down, otherwise its refrigerator aerodynamic characteristics might get you in trouble. But if the Seneca is a bit more comfortable, because apparently it was designed from inside out, the Baron has awesome performance. A lot of power, nice wings, low drag. It flares and lands a bit like a piston single: if you keep the power, even just a small amount, you will float forever. So, the secret is the same: close it during the flare and hold, hold, hold…
But the laughing Santa technique is only applicable to general aviation light airplanes. When we step up to the jets—and my learning curve jumped in a 737 right seat with fairly low hours—then the “landing by attitude” becomes the norm. Well, physics are the same for any airplane, so pitch and power will get you safe with any flying machine—that’s why we learn it on a Cessna and we learn it on a multi-hundred-tons long hauler. But when it comes to landing, that is even more essential. So, let’s suppose you don’t know that and get your GA experience and apply it directly into landing a jetliner. First things first: please keep those two reds/two whites as far as over the threshold at least, otherwise you might either leave your landing gear before the runway (four reds) or have a runway excursion at the end of it (four whites). Believe me, both things have happened with well-trained and experienced crews in real life.
The second point: the thrust. Pitch for speed, power for altitude is as true in a Dreamliner as it is in a Skyhawk. But as we get closer to the ground—and the Dreamliner has an extra push up for the amazing wings and the consequential ground effect—in the Cessna you would go idle way before you should in an airliner. The right way to do it (and the 777/787 do it automatically, since they land with the auto-throttle engaged) is to reach idle as the main gear touches the ground. The reduction starts at around 30 feet, from a threshold crossing of 50 feet. It is a Boeing, so you still have your hand on the throttle and can override it any time.
In the 737, since we land with the auto-throttle disengaged, you decide when to bring it down or pump it up, but on a normal day, you would do exactly as the auto system of the 777/787 series does. So, yes, depending on wind variations, pressure, thermals, you might have to put some extra pounds of thrust in odd moments to assure a safe—and why not smooth—touchdown. But most of the time, the power toward idle gently from 30 feet until touchdown works nicely.
Smooth touchdown is not the primary goal for an airline pilot.
And remember we talked about the Cessna approach speeds being based on its maximum takeoff weight? Well, in an airliner, they are based on the actual weight. In a Boeing 737, one knot for every 500kg or so. In a 787, one knot for around two tons. Lift goes up at the square of the speed, right? So, a light airplane will land fairly slow (in the 120-130 kts range), while a heavy one, up to the maximum landing weight, will have a Vref closer to 140 or 150 kts. And the reference speed is based on 1.3 times the stall speed. On top of that goes the five knots for the approach speed, which is going to be bled off during the flare.
So, let’s think about the Cessna again: you hold it off until you almost stall it. An airliner, quite differently, touches down with a 30% margin over the stall speed, which means it could fly still with enough angle of attack. That’s why attitude is of paramount importance; in the case of the 737 and 787, something around four to six degrees of pitch up ideally, with some margin up and down. But of course, you are eating 210 feet of runway per second so you are not really counting the degrees on your primary flight display.
So, how do we do it? Well, if you crossed the threshold at the correct speed and power setting, your pitch is right where it should be, as is your sink rate—something around 700-900 feet per minute typically. As the radio altimeter starts singing down the “fifty… forty… thirty…” song, you feel that is time to flare, and slowly but surely pulling back the yoke, you arrest that sink rate right to touchdown with the correct pitch—you are looking outside now, not to your instruments—even if using a heads-up display.
If you pull too much or get rid of the power too early, chances are you are going to either float and lose the touchdown zone, hit the tail on the ground, or both (a tail strike happens at around 10 degrees of pitch up in many airliners). If you don’t pull at all, then you land hard, with nearly the sink rate of the whole approach—what is called during the certification process “aggressive landing technique,” a very good name.
Now, since we have talked about the landing itself, how to make it smooth? Well, the bigger the airplane, the easier it is. Because of the design of some gear bogies—like the 767 or A350, with a negative tilt—they are a bit harder to grease. At the other extreme are the 747 and the A330, where you literally have more than one chance to do it right, since the aft pair of wheels is much lower than the front one during touchdown. The 787 falls in between, especially the -9 and -10, which have a nice attitude of the wheels, making it much easier for the pilot to put it on the ground softly.
The 737, since has a simple main gear, has only one chance to get it done right, but is not altogether impossible. And how to do it smooth and safely? Well, I was lucky enough to have a very experienced simulator instructor during my initial training on the 737. Coming from the 707 and the 767, he taught us the following concept: you arrest the sink rate and only then put the gear on the ground yourself. Since the airplane is in a pitch up attitude at this point, as you lower the pitch, the relative angle with the runway will decrease and touch the main gear on the ground—that, and not the whole descent of the airplane, will make the touchdown. Remember, you are still flying 30% above the stall speed.
I’ve been doing this for most of the last thousand landings, and although it does not work every single time, I have enough smooth touchdowns to not remember them all, and few firm ones to fill one hand only. With the non-negotiable aim of safety, with this technique it is possible to have a smooth landing without compromising safety, because you press the aircraft positively on the ground—getting spoilers up, autobrake on, and reversers opened nanoseconds later.
Not all landing gear systems are created equal.
The tricky part is, of course, to calculate when to stop the flare (ideally a few inches over the runway). Sometimes you do it too early and, since it is secondary to have a smooth touchdown, at any point you just let it go: keep the attitude, give it back a little if needed, and do a normal landing. Sometimes you do it too late, and a not so pretty touchdown happens. Easier said than done, faster done than said. But safe, above all.
And since the heavy plastic has this feature of sensors measuring the landing, how smooth is too smooth? Well… a normal approach occurs somewhere between 700 to 900 feet per minute. By my own experience, I’d say that any landing below 200 feet per minute feels very smooth, more in a widebody than in a narrow. Between 200 and let’s say 350 feet per minute feels normal—not especially smooth but not firm either. Above 350 feet per minute, up to somewhere close to 600 feet per minute, that’s a firm one. The aircraft can stand it, it is still safe, it just does not look pretty (or intentional, for that matter).
The hard landing concept itself depends on the manufacturer and is more G driven. On a Boeing, the pilot feeling dictates the decision to classify a landing as hard—the sensors can be deceived by lateral loads or secondary touchdowns, for example. Besides, they measure decimals only so a smooth landing tends to be 1.0 G. If you have a still decent one, let’s say 1.2, it means a 150 pound person “weighs” 180 pounds at touchdown. You see the problem? It is too much. Where is 151, 152, 153… 179, 180? Not very precise: I even got a 0.9 landing once. But the runway was kind of uphill, so yes, maybe that’s why the sensors understood that I landed with less than 1 G, as weird as it can sound.
Now, let’s take one of my last landings, a personal record: 38 feet per minute at touchdown, according to the measurement. Same technique: flare, pitch up, reduce the pitch slightly to put it on the ground. By the speed we had at that moment, this meant around 1 foot of vertical move over 120 feet of horizontal displacement. This is how smooth it was, and honestly, it felt super smooth indeed: cold temperature, high pressure, some headwind, all in my favor in a nice and sunny spring British morning. But I don’t believe these sensors for a hard landing, why would I believe them for a smooth one? I must stop looking at the numbers after each landing. It has to be only safe, after all.
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The majority of the considerations are effectively right, but, let me tell You, the others are a bit subjectives ones.
I love it! And I will always remember and adopt the term “laughing Santa technique”. Very funny!
Great read, Enderson! And thank you again for making us think.
The most interesting landings come from short body MU2s, crash it on the runway and throw it into beta; one in twenty come out smooth but I don’t know why. In a Carbon Cub if the stall warning isn’t sounding in the last 50 feet above the ground you are too fast, but it takes a couple hundred hours to learn.
Very interesting discussion to say the least and I totally get it. There are times when touching it down “like a butterfly with sore feet” seems, looks and feels really impressive but as the writer indicates, we are flying in different conditions right down to touchdown, i.e ” Crosswinds, thermals, gusts etc. Getting the gear on the surface positively but safely is a good landing. Thanks again for the article. Safe Landings!
I learned to fly on Cessna 150’s but I have found that the best way to land my Ercoupe is just to fly it onto the ground. Slow it from 65MPH to 60 MPH on final and then just pick a spot and set it down, flaring a bit right as you are about to hit. I usually aim for those big numbers at the approach end of the runway. Properly set up Ercoupes develop no lift with all three wheels on the ground, so you are not simply flying in very close formation with the runway when you touch down; you are done flying.
But 20 plus years ago there was a period when I decided to try to land it as slow as possible. And I made a bunch of rough landings, inevitably giving it a burst of power when the sink rate became that of a Lunar Module. But one day it all came together. There was 10 kts of wind right down the runway and I slowed it up as much as I could. I had the wheel all the way back and the wings were rocking gently all on their own when we touched down. I could not have been going much over 35 MPH on the runway.
As luck would have it, a guy I worked with had just parked his Comanche and was standing on the wing. The next day at the office he said, “That had to be one of the greatest landings in the history of aviation!” I responded that it was not all that great, pretty average for me. But then I went back to landing it the way I knew was best for me.
“I even got a 0.9 landing once. But the runway was kind of uphill, so yes, maybe that’s why the sensors understood that I landed with less than 1 G, as weird as it can sound.”
Interesting! I did exactly the same thing on a dry runway (no hydroplaning) flying a Great Lakes biplane on an upslope hard surface runway. My front seat passenger – also a senior lifelong pilot – commented through the intercom, “I’ll bet you can’t do that a second time.” The first sensation we were in contact with the runway was when the tail started settling to the runway without moving the controls. Flying the approach, after the flair I held the stick still and waited for the ground to come up. I don’t remember any sensation that the main gear wheels were turning!? I didn’t challenge my buddies bet, knowing Lady Luck was on my side that day.
As I watch the birds flying and landing in my backyard, particularly the hummingbirds. I have to conclude that our attempts to explain the landing process are laughable. We are at least 1 million years behind.
I flew all the Boeings except the 727 and 737 and of course the 787 and I have a lot of time in DC9’s, DC8″s and the MD11. Boeing built the landing attitude in and you were best to leave it alone…just ease the power off and hold the attitude. The Douglas was a different story. You eventually got to where you could make decent landings but for no apparent reason other than someone actually flaring, it would let you know it was still in control. The MD11 was another beast. You either loved the airplane or hated it. I loved it and the technique I used was to listen to the radar altimeter and if was counting down fast (50.40.30) then I had to do something to arrest the descent but it was coming down steady…50…40…30…20…10. At ten feet I would go idle, hold the attitude and let out a big breath of air and let the nose just sag and it would roll on. The mains are 102 feet behind you. The one absolute thing you NEVER ever did on an MD11 was de-rotate out of a harder landing. The physics of how things looked made it possible to be in a bounce and when you de-rotate you hit the nose and then the mains would slam down and after the second time it would roll on its back. Only one MD11 was lost that was not a landing accident.
The MD-11 has trully a not good history when it comes to landing. Looks to me that a small elevator and a not so well solved CG might contribute to this unforgiven characteristics, but this is a potencially uneducated guess.
I read the 1st few remarks about not having a discussion on conventional gear aircraft and was disappointed though I appreciate the author’s honesty about not being an authority on those. I have landed many taildraggers many times Including the Tailwind W10-8 that I currently own and fly. Every once in a while I get that elusive beautiful touchdown in a 3 point position where the tires barely squeak and the airplane is in a full stall. Generally that is not the case because truthfully I like the tail wheel to hit a split 2nd before the mains do it just for the extra control. I do not wheel land and I will not get into the discussion why I hear. It would be great to hear someone talk about our love to conventional gear airplanes. Now I’m going to go back and read the article.
I have very little experience in taildraggers, but it is one of my new goal in GA when I am able to resume my training in light aircraft! I hope you have liked the article.
“slowly but surely pulling back the yoke, you arrest that sink rate right to touchdown with the correct pitch” – “the power toward idle gently from 30 feet until touchdown works nicely” – “An airliner, quite differently, touches down with a 30% margin over the stall speed”. The airliner approach speed is 30% (1.3 x Vso) over stall speed as you said. How can you increase drag (pulling back on the yoke) and reduce thrust (power toward idle) and still land at 30% over stall speed?
The answer is inertia: you are approaching at Vref+5. Those five knots are bled during these 4 or 5 seconds. But the energy does not go away so quickly because of the aircraft mass – from a couple dozen to almost three hundred tons, depending on the model.
ok. Vref+5 helps to clarify because even considering inertia, there’s no way you are going to stay at a 1.3 approach speed when drag is increasing and thrust is decreasing. You are above 1.3 and I suppose the thrust is not significantly reduced until you are pretty close to the surface.
Enderson – since you opened the door and mentioned pitch and power, I guess I’ll have to ask why you say one should pitch to airspeed. The Airplane Flying Handbook recommends pitching to control vertical speed. The Instrument Flying Handbook recommends pitching to control the glide slope. Autopilots that couple to the ILS localizer/glideslope slave the elevator to the glideslope needle.
The door is always opened ;-) Well said, but you are giving as an example the system approach, not the physics behind it. The trim holds airspeed, and airplanes climb because of excess of power, not for pitch attitude. So, because of system limitations, they might couple the flight controls only, but if you do not manage your power (or an auto-throttle system do it for you), you are either going to eventually stall for lack of thrust or overspeed for excess of it. So, yes, it is pitch for speed, power for altitude, as counter intuitive as it might sound.
No – you have the physics wrong. The recommended technique (pitching to glide slope) is correct because the lift from the wing is the only force that can oppose the weight. We all know that when the pitch is adjusted to stay on glide slope (as either the pilot does or the autopilot does), that changes induced drag, so a simultaneous adjustment must be made with the power to keep the thrust and drag in balance. And trim does NOT hold airspeed. This is explained over and over when discussing go-arounds and is one of the most well-known hazards. When an airplane is perfectly stabilized and trimmed out on final, and then the pilot applies full power to execute a go-around, the airplane will pitch up too much and stall. The trim will definitely not hold the approach speed. This would occur primarily in singles where the increased propeller slipstream from the increase in power lowers the tail. In an airliner or twin with no propeller slipstream around the fuselage, the airplane would accelerate in the direction in which it is pointed.