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.
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.
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.
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.