March 23, 2015
Check out Xavion at www.Xavion.com if you do not know what it is.
OK done? All right. So all of my Xavion testing so far in REAL airplanes has been done in planes with fixed landing gear.
Here is how the system worked:
You would enter, into Xavion, the BEST GLIDE speed and descent rate. (Simply enter the best glide speed for your airplane, and the vertical speed that results with the prop feathered if possible in your plane. This would be in flight, not landing, configuration: Flaps and gear UP).
You would enter, into Xavion, the WORST GLIDE speed and descent rate. (Simply enter the high speed that Xavion recommends based on your V-speeds, and the vertical speed that results with the prop forward if possible in your plane This would be in flight, not landing, configuration: Flaps and gear UP).
So you see, for the gliding portion of the engine-out approach, Xavion knows the BEST glide you can get (best glide speed, clean aircraft, prop feathered if possible) and the WORST glide you can get (higher-speed descent, clean aircraft, prop forward if possible). Since Xavion knows both the best and worst glide yo can get, it simply designs an approach that is exactly halfway in between those two glide ratios! This way, the margin for error is maximized, and the chance of being able to fly the planned approach, power off, even if MORE stuff goes wrong, is maximized. Cool!
Now, for the round-out to touch-down, which ALSO matters:
You would enter, into Xavion, the LANDING GLIDE speed and descent rate. (Simply climb to a dafe altitude, configure for LANDING (Touch-down speed! Power-off! Gear and flaps FULL DOWN! Prop forwards!) and enter the vertical speed that your airplane delivers. From this forward speed and descent rate, Xavion determines your glide ratio, and from that Xavion can know for certain exactly how fast your airplane will DECELERATE during the round-out and touch-down! Cool! This information is needed for Xavion to not only guide you down TO the runway, but through a round-out and flare that will bring you to the touch-down at a nice comfortable landing speed! This, too, is part of the energy management. And when Xavion designed this round-out, it assumed that your application of flaps, gear, and forward prop would be brought in smoothly throughout the round-out. Start the round out clean. Bring flaps and gear and prop in through the round-out. Touch down dirty, configured for landing, at a nice touch-down speed. (Note: The round-out in Xavion starts at about 500 feet or so, and takes about a minute or so to complete to touch-down: It is a very smooth, gradual round-out that is designed to give you time to configure the airplane for landing and gently bleed off speed as you raise the nose and transition from gliding to landing flight. Interestingly, this round-out is NOT designed to start at some ARBITRARY speed or altitude, though! Instead, it is designed to be just the right length of space and time to bleed off your excess gliding speed and slow to landing speed as you raise the nose smoothly from gliding attitude to landing attitude, while bringing in the flaps and gear smoothly throughout this process. So you are given the length of time and distance that is appropriate for your particular airplane to dissipate this energy, which Xavion computes by knowing both your gliding and landing speeds, and thus the amount of energy that you have to lose to slow from gliding speed to landing speed, and the drag that will be on your airplane to dissipate that energy, which it also knows form the three gliding descent rates that you entered as explained above. This works out to be a round-out starting at about 500 feet above touch-down in a plane like a Columbia-400 or Cirrus or the like, and would of course be closer in for a Cessna 172, and farther out for a TBM-850).
So, it sounds nice, doesn’t it?
And it works perfectly, in all of the thousands of tests that I have run in the simulator, and hundreds of tests I have run in real airplanes.
BUT, all of the tests that I ran in the REAL airplane were run in FIXED-GEAR airplanes. Retractable gear tests were run thousands of times in the simulator, so they were mathematically proven to work, but my actual in-flight testing was limited to fixed gear planes. As I would fly my Columbia-400, power off, into that round-out at the end of the glide, I would smoothly bring in flaps as the flight path smoothly rounded out from descent to touch-down, and always touch down 750 feet short of the center of the runway (the PERFECT place for a plane that takes 1,500 feet to stop) at 75 knots or so, pretty as you please, after a smooth flaps application made during the round-out. It was perfect.
Then I got my Evolution, a retractable-gear airplane. This airplane descends about THREE TIMES AS FAST with the gear down as up!
Now, the math in Xavion still guaranteed that the energy-management for touch-down would work!
The drill in the retract-gear-equipped Evolution was the same as always: Fly the glide down to the airport, power-off, clean, at a speed and decent rate that are halfway between the best and worst glide for the airplane. Then, during the round-out, bring the flaps and gear in smoothly throughout the round-out, and touch down dirty at 75 knots, 750 short of the center of the runway, and use about 1,500 feet of roll-out with modest braking, stopping 750 past the end of the center of the runway, thus touching down at the perfect place and perfect speed and perfect configuration: Just what you want after an engine failure in flight!
BUT, do you notice anything in the sentence above that makes the hair on the back of your neck stand up a little bit? Maybe not, but if you fly a Lancair Evolution, then you just might notice something that makes you nervous: You are extending the gear DURING THE ROUND-OUT… wow… that is really at the last second, isn’t it? Sure, the round-out starts at 500 feet, but if you bring the gear in halfway through the round-out (which is recommended since that is most consistent with a smooth application of gear and flaps bing brought in smoothly throughout the round-out) then that means you are actually lowering the landing gear at about 250 feet above the touch-down. Wow! That is REALLY last-second! Now, lowering the gear so late might SEEM just crazy, since we are CONDITIONED to lower it at 1,000 feet or so, giving us eons of time to make sure it is down, check it three times, try an emergency procedure to extend it if needed, etc etc etc. But, in a REAL emergency, the most important thing is that you glide well enough to make it to the airport! Thus, lowering the landing gear at the last second becomes preferable. After all, if your engine failed, and you go to lower the gear, and it does not come down, what are you gonna do? Miss the approach? Yah, right. I remember watching the Space Shuttle land, and watching the gear come down just MOMENTS before touch-down! I thought: “WHAT? Why do they wait til the last second to lower the gear!?!?”… and then I thought it through: The Space Shuttle was a glider, so if the gear broke and would not extend, what were they gonna do? Miss the approach, climb up to a traffic pattern east of the airport, fiddle with the landing gear, and then come back to the airport for another try? Yah, right. Gear or no gear, in a glider, they are touching down either way, so why bother trying to lower the gear any earlier? If it works, it works. If not, you touch down on your belly. You are landing either way, so why both with the gear sooner? You won’t have time to fix the problem either way, so there is simply no benefit to trying to lower the gear earlier.
So all this is true of the Space Shuttle, and of your airplane as well if you lose power: Your safest move is to leave the gear UP until the last moment, because if it does NOT come down, you are not going to have time to diagnose and solve the problem even if you DO try to lower it a bit earlier, and touching down on a runway without landing gear will usually not HURT you or your passengers or anyone else on the ground, so your safest move is usually to leave the gear up just to make sure you MAKE THE AIRPORT, and then lower it at the last moment once the landing is assured… since getting down on the runway is most important by far from a safety standpoint… doing it with the gear down is secondary.
And all of this is mathematically true and tested thousands of times in the simulator, simulating real emergencies… but what about the case i have not mentioned too much: The case where you are PRACTICING using Xavion In the real airplane? In that event, do you really want to be lowering the gear at just a few hundred feet above touch-down? When you are PRACTICING using Xavion (fun and useful!) do you really want to subject yourself to this risk? It certainly leaves very little time for the gear to extend, very little time to CHECK the gear, and if the gear does NOT go down, very little time to execute a graceful missed approach. And, in PT-6-power aircraft like the Evolution, there is ANOTHER skeleton in the closet: If you are flying the round-out with the prop feathered or near feathered because you wound up a bit lower or slower than you thought you would, then you will NOT be able to quickly apply power! Why? Well remember: The Pratt and Whitney PT-6 engine has ZERO connection between the compressor and the propeller! ZERO! Instead, the compressor simply compresses air, and the compressed air squeezes through a turbine that is connected to the prop! No connection at all between the prop and engine! So, if you have the prop feathered for the glide, then it is only turning a few hundred RPM. If you suddenly decide you want to miss the approach: IT AIN’T GONNA HAPPEN. The reason is that with the prop hardly turning, and the compressor at idle, there is hardly any airflow at all through the engine, and since airflow IS the torque coupling between the engine and the prop, there is almost no torque coupling between the engine and the prop! As well, with the prop up at its’ feathered pitch, those blades are NOT at any sort of angle or RPM that will generate any thrust to speak of. So, while the Evolution glides almost like a glider when clean and with the prop feathered, you can NOT just push the throttle forward and miss the approach! FIRST you have to advance the PROP CONTROLLER out of idle and move it at least most of the way forward to get some decent RPM. The prop governor, on seeing this, will respond by flattening the pitch and increasing RPM… BUT THIS TAKES TIME! The pitch comes down slowly at first, and the ROPM only increases slowly. Then, at some point, the prop “catches”, and the RPM ZOOMS up to the commanded RPM… but this moving from feather to redline RPM takes precious time… certainly more than 10 seconds when I move the RPM controller carefully to avoid any chance of a sudden over-speed. Then, once the prop is forward and spinning, you have to advance the throttle: And that compressor does NOT spin up instantly! It takes some time to spin up to the 39,000 RPM (!!!) of full power. And not only that, but on those PT-6 engines, you NEVER push the throttle forward all the way! This would over-torque or over-temp the engine for SURE! You need to advance the throttle SLOWLY to hit a proper take-off or go-around torque setting… shoving the throttle forward quickly is likely to result in an over-torque. And, this being an Evolution, there is MORE to the problem (man, these are fascinating airplanes): Evolutions have so much power that their ailerons and rudders can only ABSORB a limited engine torque at low speeds! In fact, ALL Evolutions take off at only two-thirds power, because if we went to full power for take-off, the torque from the engine would be so strong as to possibly over-power the rudder or ailerons if the pilot was slow in his control, thus rolling the airplane left! So imagine this scenario: You decide to miss an approach in an Evolution. Your speed is low and you are low (possibly the very reason you decide to miss!) and you suddenly jam the throttle forward. At first, nothing happens: The compressor does not spin up instantly, and neither does the prop if you were not already at redline RPM. As you wonder why the engine is not responding, you pull back on the stick some more, subconciously trying to avoid the ground, and actually bleeding off precious airspeed. Now, all of a sudden: BAM!!!!!!! That compressor spools up, the prop pitch opens to absorb the sudden engine torque, and you suddenly have a BOAT-LOAD of torque (Could easily be 1,500 foot-pounds: The weight of a CAR hanging on a 6-inch lever-arm off the side of the plane, and the in the -42 engine could exceed 2,200 foot-pounds, the weight of a small car on a ONE FOOT lever arm). When that torque hits you like a wave, if your speed is too low to absorb it, and you are late on the controls to counter it… well… the plane will roll and even yaw left… perhaps strongly. Perhaps very strongly. I’m trying to be politically-correct here in explaining what happens when you roll and yaw an airplane close to the stall speed, at very low altitude.
So, you want to miss the approach in a PT-6 powered airplane, and especially a small, light, strongly-powered airplane like the Evolution, you want to do it smoothly, gently, slowly, and well in advance. Not suddenly at the last second.
So we know that we want to miss the approach slowly, smoothly, and well in advance in a PT-6 airplane. We also know that if the prop is feathered and the engine at idle, it is going to take a long time to simply EXECUTE the go-around, because you will have to push the prop forwards and WAIT for it to spin up, and then push the throttle forward and WAIT for the compressor to spin up. And only ease the throttle forwards partway, carefully setting it to not over-torque the engine, or the airplane if your speed is low! All of this says the same thing: Don’t position yourself so you have to go around at the last second, and if you HAVE engine power (you are not in the engine failure scenario) then don’t fly the approach with the prop feathered: It will make MISSING the approach problematic at best.
Finally, all of this is to say: When PRACTICING power-off approaches using Xavion, do you really want to lower your gear at the last moment? If the gear does not come down, it may simply be too late to execute a really smooth, safe, missed approach because of the time required to smoothly do so. In the REAL emergency, there IS no missed approach, so none of this is an issue: A feathered prop and last-second gear-deployment are fine, and even advisable! So Xavion has historically guided you accordingly. But, since we want to safely PRACTICE with Xavion as well, where we MIGHT miss the approach if our practice approach does not go well, we want to set up Xavion in such a way that a last-second missed approach is NEVER called for.
So how do we do this?
Well, we clearly need to get the gear down EARLIER, so if it does NOT properly extend, we have plenty of time to smoothly and safely climb away.
So how do we lower the gear earlier, and STILL MAKE IT TO THE RUNWAY? Remember, the Evolution glides THREE TIMES WORSE with the gear down (and all retract planes glide somewhat worse with gear down).
Well, assuming that we do not want to approach the runway with boat-loads of speed (We don’t! We need to be slow enough to lower the gear! To lower the flaps! And to THINK about what is happening as it happens!) then there is only one answer: WE nee to approach the runway with some excess altitude. How much? Xavion now gives you 50% TIME_BUFFER above what is used for the round-out.
Here is what I mean by that: IF the round-out takes 60 seconds for your airplane (because Xavion computes the speed that you have to shed to get from gliding speed to landing speed, and the deceleration that your plane will provide to do so, will take, in fact, 60 seconds), then Xavion will add an extra 30 seconds of glide BEFORE the round-out for you to lower the gear. And here is what is crucial: This extra 30 second glide assumes that you lower the gear at the BEGINNING of this 30-second buffer, not the end of it, and Xavion STEEPENS your glide for this gliding segment, so as you lower the gear and increase your aircraft drag, you can gently lower the nose to hold your current speed down to the round-out. AH-HA! You now have an extra 30 seconds BEFORE the round-out to lower the gear, lower the nose because of the extra drag, and approach the round-out with the gear already down and checked! NOW you can focus on a nice, smooth, round-out, adding flaps gradually throughout the round-out, and gently adding power to miss the approach if needed, since you are not trying to worry about lowering and checking gear: That was all taken care of before you even GOT to the round-out. NICE!
NOW, I know that this all SOUNDS complicated, but it actually is NOT! Why? Because Xavion has only one way of guiding you: BY presenting 3-D hoops in the sky! You just fly through them! But now, if you have a retractable gear plane, then you might notice that the hoops STEEPEN a bit before the round-out, giving you the perfect place to lower your landing gear, and then lower the nose as well as the gear comes down. THEN start raising the nose and lowering flaps throughout the round-out, which you easily see as the hoops simply gradually flatten out to smoothly touch the runway! So all you have to do is follow the hoops.. easy as can be! Xavion has planned everything out that I just discussed, so you do not need to think it all through in the middle of an emergency.
OK, I made Xavion do ONE MORE THING to make this easy: At the very start of this transition from glide to landing, I made Xavion show you hoops that are RED instead of MAGENTA. So, as you are flying through the 3-D hoops with the engine either dead (real emergency) or at idle (you are wisely practicing the system), you simply maintain the glide speed recommended by Xavion while in the magenta hoops, aircraft clean. Then, as you approach the RED hoops, that is when Xavion expects you to lower the landing gear if you have retractable gear, and lower the nose as a result if you have retractable gear. About 30 seconds or so later, the hoops will start to round out to approach the runway smoothly: That is when you ease the nose up to follow them, and ease in your flaps as desired to touch-down where and at the speed you like.
WHEW! OK, that may seem like a lot of theory, and a lot of math, and a lot of uncertainty, all of which is a bit scary, but it is just like a will or an insurance policy: Scary to contemplate or not, all of it is designed to do the same thing: Have a PLAN for the worst case that is carefully thought out in advance, so that when the time comes, the actual execution of the plan is simple and straight-forwards: Follow the magenta hoops in gliding configuration, at the speed Xavion recommends (halfway between best and worst glide to maximize the margin for error and thus your ability to FOLLOW the plan even if OTHER stuff goes wrong at the same time), fly that speed and configuration (adjusting prop RPM for engine braking as needed, if possible) until you hit the red-colored gates. Then lower the gear if you have it, and the nose as a result, and then smoothly raise the nose and add flaps as the hoops flatten out. When I practice this, the result is almost breath-taking: A perfect smooth, normal-attitude, normal-speed touch-down, half a roll-out length short of the middle of the runway: No drama at all… and this after we lost power at some random location far from the airport! And, now, we are configured to be able to do this in fixed or retractable-gear planes, with a gear extension that is done a bit in ADVANCE to give you the time to PRACTICE safely a well.. even in a plane like the Evolution, in which gear-extension and go-arounds are best done well in advance.
So, this is the next level for Xavion: Being set up not only to handle real emergencies, but safely PRACTICE as well, in a very wide range of aircraft!