C-17s land on their backside...

[PRB]

I got to fly Boeing's C-17 simulator this past week. Full motion type. It was a very interesting experience. During landing at Honolulu, the instructor pilot made a strange comment. He said the C-17 lands "on the back side", like that was an important point I should know. So I asked him what that meant. He said it means you don't pull or push on the stick, at all, during the approach, just side to side for line up. And, at the point in the landing where a "normal" airplane would be flared, in the C-17, you add power, instead of reducing it. None of that made sense, but I tried to do as I was told and not push or pull the stick. Of course it took about two seconds before I pushed it, and immediately corrected myself, saying "oh dear! I pushed it!!" He thought that was funny. The IP worked the flaps and power, coming up on the throttles as we came over the numbers, and we landed. Weird.

When I got home I Googled this business of "front side and back side". The first hit I got was some site where air force pilots were talking about transitioning from flying C-5s to C-17s. One "new guy" posted "I've heard horror stories about back side flying, what should I do to prepare for the C-17?" Most responses from C-17 pilots were along the lines of "don't worry about it, it's easy and you'll pick it up quickly". But nobody said what it was! Back side of what? More Googling…

Turns out it's the back side of the power curve! Basically, the explanation is this: Pulling on the stick causes an increase in both lift and drag. Above a certain speed, pulling on the stick lift increases more than drag. As a result, the nose goes up and so does the plane. Below a certain speed, the reverse is true. Pulling on the stick increases drag more than lift. Now the nose still goes up, but the plane goes down. Now you're on "the back side"!

This also explains the landing technique of using power to adjust glide slope, and pitch to adjust airspeed. When you're landing, you're slow, where this transition from front side to back side occurs. But if that's true, then the C-17 landing technique should not be unusual. Don't all airplanes land this way? Apparently not. From the stuff I found on the "Interwebs" carrier planes fly approaches "on the back side" because in order for the hook to catch a wire, the plane has to be at an AOA that requires flight at a speed on the "back side" of the power curve. But Air Force planes, and GA planes, fly approaches on the "front side". Except for the C-17…

In the C-17, because they hold the same pitch attitude throughout the approach, they appear to use power to arrest the vertical velocity at the last moment. You're supposed to put the HUD velocity vector "on the numbers" during the approach, then, when it's time to flare, you don't, but add just enough power to put the velocity vector on the far end of the runway.

So next I went to FSX to see if I could explore this mysterious region of slow speed flight. I'm not sure FSX models this correctly. I tried several planes, and I always seem to reach stall speed before I get to a point that pulling back on the stick does not result, at least for a while, in a change in vertical velocity. And this raises into question the entire real world technique of using pitch to control airspeed during the approach, in FSX. It sort of works, but not really, since it always also reduces vertical velocity.

Some reading material:

http://joeclarksblog.com/?p=771

http://flighttraining.aopa.org/magazine/2002/January/200201_Features_Behind_The_Power_Curve.html

[StormILM]

One of the first things my Instructor in Primary taught me: "Pitch Controls Airspeed, Power Controls Sink Rate" (specific to Approaches). Sure, you can push the power up and speed up, pitch the nose up or down the gain/lose altitude but his point was the settle the aircraft into a stable slow speed stable AoA regime (aka, Approach Configuration). It changes the control dynamics from coarse handling to finesse based handling. Coarse handling at slow airspeed/typical approach altitudes can and will get you in deep trouble very quickly with most aircraft. I remember MCA (Maneuvering at Critical Airspeed) training. It was fun to have that C150 dirtied up with full flaps, max AoA and making shallow standard rate turns at 42kts indicated! My instructor showed me what would happen if you over-controlled the aircraft (making ham-handed inputs), we stalled and fell into the classic stall-spin entry that a number of unlucky pilots encountered low during approaches. It was a good experience and one that taught me the finer points of flying.

[joe bob]

"It sort of works, but not really, since it always also reduces vertical velocity."
In a stable approach on speed if you pitch for an airspeed change then you will get it, you just have to anticipate it. you have to wait a beat for the airplane to settle down to it's new trimmed airspeed. if this new airspeed gives you a rate of descent that is not what you need then it is time for a power change.

[Portia911]

Interesting read - thanks for sharing your research, Paul.

[scotth6]

Yeah thanks for sharing and for the links. That is very interesting reading. Great to learn some real life facts about the C-17; one of my favourite planes.

I must be picking up some bad habits being a FSX pilot for so long. I know I don't do things by the book or by the numbers, but I always manage to land. I try to set up a stable, nose up attitude while on approach, but I am always diving for the runway and catching the glide slope from above. I am a bad, bad pilot!

Cheers,

[PRB]

Yeah thanks for sharing and for the links. That is very interesting reading. Great to learn some real life facts about the C-17; one of my favourite planes.

I must be picking up some bad habits being a FSX pilot for so long. I know I don't do things by the book or by the numbers, but I always manage to land. I try to set up a stable, nose up attitude while on approach, but I am always diving for the runway and catching the glide slope from above. I am a bad, bad pilot!

Cheers,

Yeah, I'm a "bad" pilot too! During my landing, the real pilot, in the right seat, kept telling me I needed to come left for lineup. I knew that, but I was only a teeny tiny bit off to the right! Kind of like all my FSX landings, good enough... Then I over corrected to the left, of course. It's very sensitive to stick inputs.

[hairyspin]

An intriguing read, thanks! I tend to land on my backside, so perhaps this will help land on the wheels instead.

[Dev One]

Sounds similar to doing a 3 pointer on a taildragger. My father taught me that in the approach power = distance, pitch change for speed. Obviously this can change from one plane type to another & this was in his Percival Proctor. As for Flight Sim to get the same effect I think one would need to change the curve of the Cl vs AoA approaching the stall & possibly then see if there is an added curve applicable for flap deflection. There is no curve for drag alone, it seems to be calculated from Cl vs AoA.
The late Eric (Winkle) Brown who did the first Vampire jet landing on a carrier, flew the approach & landing on the back of the drag curve, utilising the speed brakes, which meant that he could keep the RPM high in order to provide almost instant thrust if he needed a go around, I wonder if that was the initial thought for the C17?
Keith

[scotth6]

Interesting thoughts Keith. I also wonder if it has something to do with the C-17's STOL capability. I tend to use a higher power setting with speed brakes deployed on approach for some military aircraft in FSX and DCS, and as soon as you chop the power the aircraft will slow quite suddenly. In fact I just did it a minute ago in the A-10C; but I really had the brakes out because I was high and fast again.

Maybe having a higher power setting also helps with the reverse thrust stopping power, which the C-17 makes pretty good use of.

Cheers,

[txnetcop]

Piper Pacer is pretty much the same way when it comes to approach and landing...great post Paul!
Ted

[roger-wilco-66]

"Stick and Rudder" by Wolfgang Langewiesche explains these principles very well. For me one of the best books about the basics of flying an aircraft.

Cheers,
Mark

[PRB]

"Stick and Rudder" by Wolfgang Langewiesche explains these principles very well. For me one of the best books about the basics of flying an aircraft.

Cheers,
Mark

Thanks Mark. Will check that one out.

Just tried the Milviz T-38. So far it comes the closest to being able to duplicate this back side behavior, as described in the linked articles. But even this plane, when increasing AOA, without additional power, results in nose coming up and higher sink rate, occurs at about the same speed as the stall buffet begins...

[airattackimages]

Yep, the C-17 lands power on.

You can do this with any aircraft with copious amounts of power.

[Sundog]

Here's a nice graphic of it;




What will help you understand it is to know that the curve called "Power Required" is the total drag curve. As you get slower and slower, you need higher and higher alpha to maintain lift. Higher lift means higher drag (induced and due to flow separation and higher frontal area relative to the velocity vector).

It isn't a problem unless you are close to the ground, such as on landing, and get to such a high alpha/drag setting that you have more drag then power available. The only way to correct that is to add more power, which you don't have, or reduce drag. To reduce drag you usually have to lower the nose, which means lower alpha, which means lower lift, which means you don't have altitude to gain airspeed, which means you are going to crash.

This is what happens when you fly into that region in the upper right hand corner of the graph; which was also probably compounded by the fact that not many pilots know at high alpha the ailerons control yaw and the rudder controls roll. F-100 get's behind the power curve.