Weight and balance is a very big deal on some aircraft and it is possible to load fuel, cargo, or people so the aircraft simply will not fly. See: CL NV for an example. On other aircraft, such as our oft-used example Gulfstream G, aircraft design makes it harder to find the airplane un-flyable but it still needs to be considered. See: Weight and Balance. In either case, you should have complete understanding of the concepts and a very good idea of where your airplane's range of acceptable centers of gravity lie. An aircraft with a conventional tail-mounted horizontal stabilizer uses that stabilizer to exert a downward force.
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The bottom line up top: It takes energy to flare and the only control of that energy you have is airspeed. Assuming your gross weight, glide path angle, and environmental conditions have already been set. We can measure the energy we have available if we have an Angle of Attack instrument, realizing that the difference between 1.
Armed with this knowledge, we can improve our odds by adding airspeed, but not too much. You still need to stop the airplane once you've landed. While we are on the topic of landing energy, we should also talk about windshear. One of the most valuable resources I've ever had in a cockpit was a Military Airlift Command flight engineer. Not only were they invaluable when it came to fixing things that were broken, they tended to be excellent cockpit procedure stewards.
Their manual, AFM , is filled with techniques still applicable today, including what we in the Air Force used to call the Minimum Groundspeed Technique , something that is electronically considered in some Airbus aircraft and is worth understanding in any airplane.
A big thank you to Andreas Horn for the help with the engineering concepts here. Much of what I am presenting here is "above my engineering paygrade," and I relied on Adreas' help. How much surplus energy do you have when you approach the flare? I think one way to look at that is to examine the ratio of lift during the approach versus the maximum available lift.
Photo: C L at two angles of attack, example aircraft, from Eddie's notes Click photo for a larger image. Given this, we can write two equations, one for Lift at reference speed and the second for Lift at the stall, or maximum lift:.
Doing the math, we come up with the ratio of lift generated versus the lift available needed to maintain glide path without losing speed:. Notice that 0. So that's how much energy you have by virtue of your airspeed. How much energy will it take to flare? There are several kinds of energy working on the aircraft descending on a glide path at a steady airspeed:. An engineering fundamental some of us have this tattooed on our arms is known as The Law of Conservation of Energy:. A pilot with considerable experience doing all this safely will tell you that you want to fly the airplane onto the runway, touching down still with some vertical speed.
This has several advantages:. We know intuitively that the issue here is we have something called momentum heading into the runway and we need to break that momentum. The formula for momentum is:. We can isolate the momentum into vertical and horizontal components. Of course in our discussion of the flare we are interested in the vertical:. For the purpose of examining the flare, we'll call F the force needed to flare.
We also know that, thanks to Newton's Second Law of Motion :. We are ready for our example numbers. Finally, the acceleration due to gravity, g, is 9. From this we conclude we need to increase our Coefficient of Lift by 10 percent to cut our descent rate for the flare. It is intuitively obvious to most pilots that there is a minimum acceptable speed to arrive at the flare. Not so intuitive, perhaps, is that there may be two possible thrust combinations for that particular speed.
We need to think about keeping ahead of the power curve. But what does that mean? You can't really measure thrust directly outside of a test stand because thrust changes with velocity and how would you measure it without something for it to push against?
That's why jet engines tend to be rated in "static thrust," that is on a test stand, not moving. But you can measure drag and when an aircraft is in level, unaccelerate flight, drag equals thrust. You can plot the thrust required to maintain level, unaccelerated flight versus airspeed and come up with a chart that will show you there is a speed where the thrust required is at a minimum, Point B on the chart shown.
This is your maximum endurance speed. At any speed above Maximum Endurance, you are in the region of normal command, where the relationship between your thrust levers and airspeed is as you would expect. At Point C on the chart, for example, pushing the thrust levers forward makes you go faster, pulling them back makes you go slower.
Further, the new thrust required setting will be as you expect. Let's say you are at Point C and you want to add 5 knots. You push the thrust levers forward, you accelerate, and when you get to your target speed you retard the thrust levers, but end up at a new thrust setting which is higher than where you began. Perfectly normal, hence the name, the region of normal command.
Another benefit on staying on the normal side of the power curve is it promotes airspeed stability. If a gust of wind or other environmental factor causes the aircraft to accelerate, it will be too fast for the thrust required and will tend to slow down.
Conversely, if the disturbance causes the aircraft to decelerate, the thrust level will be too high for the existing airspeed and the airplane will tend to accelerate. At any speed below Maximum Endurance, you are in the region of reversed command, where the relationship between your thrust levers and airspeed is unexpected. At Point A on the chart, for example, pulling back on the thrust levers will cause you to decelerate, as before, but to maintain the new speed you will need more thrust than where you began.
The danger, of course, is you may not have enough thrust to maintain the new speed and the only way to accelerate will be to decrease the angle of attack. Of course we have neglected a host of variables, such as idle thrust from the engines and the ground effect of the wings.
Remember, also, that we have other "actors" in the energy equation:. But like our examination of the block and tackle pulley system, ignoring these smaller effects allows us to make decisions on the larger impacts. So here goes. That takes us to 0. So your flare rotation will "cost" you in terms of lift, but you know that because simply rotating the nose will result in a loss of airspeed.
The amount of airspeed loss will be mitigated by the idle thrust CE , but for most airplanes it is a losing battle. So the lesson here is that you don't have a large margin to fool with and the airplane will eventually end up on the ground with less than the needed airspeed to maintain flight. If you use up that margin while still several feet in the air, the result will be a landing not under your precise control.
We've seen that the flare moves us up the C L curve about ten percent and that we start around 0. The aerodynamic stall occurs around 1. A truism among aeronautical engineers translated to pilot-speak may be "you need to keep on the straight part of the C L line.
Of course this is an example aircraft and yours may exhibit these characteristics at different angles of attack. But you cannot ignore the need to keep your AOA in a safe zone prior to initiating the flare. Getting too slow also risks venturing into the region of reversed command, and as we have already seen, we want to avoid getting behind the power curve.
Airspeed, as your very first flight instructor told you, is key. Of the variables in the equations covered so far, the only one you have real control over is the aircraft's velocity, v. Recall that we had 0. The aircraft manufacturer did the necessary tests to make sure the engines put out sufficient thrust CE to complete the flare but not so much thrust as to adversely impact your landing distance. But what if you encounter a gust of wind or other environmental factor that takes away some of your V REF.
Now what? How will adding 5, 10, or 20 knots impact that margin? If you say, "In the event of a windshear, I will go around," you probably fall into one of the following camps:. If you aren't sure about that, how often have you flown into Teterboro to hear windshear had been reported as well as a gain and loss of 20 knots on final? In other words, a typical day at Teterboro. The point is that there are gusty winds within your capability that can be handled by adding to your approach speed.
But there are also gusty winds that exceed that capability. More importantly, if that windshear happens to be a mircroburst, the go around could be the only thing that can save you. So with that as a caveat, here is a procedure still in use by some operators and even at least one manufacturer's FMS. I learned the minimum ground speed technique for dealing with wind shear early on. Our flight engineers could zero in on the aircraft's ground speed as if our lives depended on it.
It resulted from an intense analysis of adverse weather airflows, motivated by the loss of several aircraft, and made possible by current weather technology. As you may know, an aircraft's ability to maintain lift is dependent on aerodynamic flow, airspeed, and movement within an air mass.
Jet streams, independent air mass movement, and airflow in and around thunderstorms provide an environment where an aircraft can almost instantaneously transition from one air mass to another. This is wind shear. The effect of wind shear is similar to the effect of wind gusts, except it can be much more severe. It can increase or decrease airspeed until engine thrust has no opportunity to reestablish the proper airspeed within the new air mass.
Further, it can increase or decrease airspeed by the difference in velocity between the two air masses. At high speeds, the aircraft could exceed its maximum design airspeed limit, or at low speeds it could stall. This is a valid technique for dealing with a shear caused by one air mass sitting atop another.
It should not be employed when dealing with convective activity, especially a microburst, where the magnitude of the speed loss or gain can be unpredictable. Let's assume that an aircraft is on final approach at a safe margin above stall speed. Further assume that we have a knot headwind on this approach and that the aircraft is flying within this air mass at knots indicated airspeed. If this aircraft transits a wind shear into another air mass that suddenly gives up the knot headwind, the indicated airspeed instantly would drop from knots to 75 knots and the aircraft will stall.
In preparation for transiting this wind shear, we increase approach speed by the amount of the predicted loss. After penetrating the shear, airspeed will immediately reduce to the approach speed.
Weight and Balance Principles
Minimum Ground Speed Technique