Density Altitude Landings

//Density Altitude Landings

Density Altitude Landings

“Ken, let’s start slowing this thing down .“ We had just touched down at Holbrook, Arizona in “Ole Green”, Ken’s beloved 450 Stearman. The field elevation is 5250 ft., the temp. 103 deg., and we were rocketing down the runway ( 90-100 mph true, maybe?). We had plenty of runway; the airplane was nice and solid in a 3 point attitude. And I didn’t like it .The airplane wasn’t decelerating at all, Oh and we had landed downhill, it just felt like we could have moved all of the flight controls to the stops with zero response.

Ken slowed the airplane, and taxied to the ramp without incident. We had both just gotten out of the airplane when we noticed something. The air wasn’t just still; it was so thin it was damned near nonexistent.

Most pilots will remember density altitude takeoff procedures and theory from their flight training, but few were ever shown actual or simulated density altitude landings . Density altitude takeoffs are easy to simulate; density altitude landings are nearly impossible to simulate. As a result, almost everybody has learned to accomplish density altitude landings by baptism of fire. Congratulations to the flying public, there are relatively few accidents or incidents due solely to high density altitude landings.(We will discuss density altitude takeoffs another time.)

Let’s turn our attention to how the physics of thin air particularly affect our” oh so easy to land” Stearmans. First, remember it is not just the performance of the engine that is diminished when those molecules are spread out. Airfoils are also affected. The propeller and our wings suffer. The wings simply will not produce the same amount of lift at a given angle of attack and speed when the air is less dense. During a landing approach, increasing the angle of attack is going to increase the drag, resulting in a higher descent rate, never mind approaching a stall in extreme conditions. Consequently, increasing the airspeed makes the most amount of sense. So, how much? If we don’t increase it enough, the resultant high rate of descent will result in a hard, possibly bounced landing. If we increase our approach speed too much we will float excessively, possibly run out of runway, and expose the airplane to the risks of higher ground speeds during decreased control effectiveness.

We all have a favorite flight attitude and resultant indicated airspeed we like over the runway threshold that results in those silky smooth landings we all make from time to time. Both the flight attitude and IAS are going to be different during an extreme density altitude landing. How will we know the best combination? After all , we want the smoothest landing possible in extreme conditions, the go around can be a real bitch.

Stalling the airplane during those conditions at that weight and chosen landing power will give us the indicated airspeed and attitude for that given airplane every time. We simply add whatever margin of angle of attack and resultant airspeed we like to what we observed during the stall we performed at that safe altitude above the ground and outside the traffic area on our way into the airport. We don’t even have to full stall the airplane, just the incipient stages of buffet and control slackness will do.

Wait a minute Tony, my favorite female passenger is not going to like this. You should also hear what the ATC controller, formation leader, and my wife thought about the idea… Enter some flying homework (our favorite kind) It is safe to say everybody knows what works for them when their home field has warm temperatures. Preselect some altitude /OAT combinations that will give you graduated density altitudes starting at 2-3,000 ft. up to 6,000 ft above the density altitudes you typically encounter. On a nice temperature inverted day, let’s climb out towards our practice area. During the climb, note the altitudes that give us our target density altitudes. Climb 1,000 ft. above the last target altitude, and establish cruise configuration. Simply fly an imaginary pattern to landing at every one of your target density altitudes. Establish downwind, and then make full base and final legs to your “landing” at that particular target altitude. Full base and final legs give you some time to experiment with the glide attitude to find what feels right. When you “land” at that altitude, note the attitude and airspeed at the moment of “landing”. Mentally note the attitudes at each altitude landing, and jot down the IAS at each target altitude landing. You may not notice a big difference incrementally, but there will be some difference between your highest and lowest altitudes. Make sure to “go around” from each landing without losing any altitude.

Count out how many seconds it takes at each altitude to accelerate the dumb old thing to a speed at which it begins to climb. Experiment during the climb to find which attitude/ airspeed combo yields the best rate of climb. Those of you with fixed pitch props will find this closely tied to the greatest RPM you can make. Remember to lean for best power, keep an eye out for other traffic , and not to fixate in the cockpit during our data gathering . When you get back to the hangar, assign ideal approach speeds for each stall “landing” speed you collected at each altitude. This now gives you a target airspeed for your approach for various density altitude airports. (Most ASOS now broadcast the density altitudes above a preset level for that airport) There may not be much difference at all between the I.A.S. over the range of altitudes, but you will have noticed some difference in the attitude. For those of you interested, dividing the true airspeed by the time it took to accelerate to climb speed would give you the distance it would approximately take to go around.( I see you smiling Jack Davis) Some of you may decide to just approach at the best rate of climb speed for that density altitude, eliminating any acceleration time lag in the event of a go-around. Of course some on the fly adjustments will be made during actual D.A. landings to allow for gusts, weight, etc. The idea here is to have seen what your airplane behaves like at those higher altitudes in a relaxed, room between us and the ground environment, instead of finding out staggering barely above stall speed inches above the rooftops and trees.

How is landing distance affected? I have yet to see an official published Landing Distance/ Balked Landing chart for the Stearman. Intuitively, we know the landing/go-around distances are going to increase with density altitude. Here is a tip to take some of the apprehension out of the rollout. While on the downwind leg to landing , make a quick study of prominent, easily recognizable from the runway landmarks (hangars,trees,tie- down areas,control towers,etc.) that we are going to use as runway distance markers during our rollout. Some rural airports are not equipped with distance markers, and skinny runways may not allow that slight little yaw at high speed to see where the end of the runway is. Pick the first marker that is the safest least distance for a goaround to clear any obstacles off the end of the runway. Our last marker is one that is ideally our” end of runway “marker. In event of a long rollout (allow formation elements to recover, blocked taxiways) this marker ideally gives us enough room to comfortably stop from a high speed taxi.

Wheel landings or three point when it’s high and hot? Do whatever you are the most comfortable with. Extreme density altitude landings usually will have gusty conditions with crosswinds. Smoothness and control during the rollout are key. Any crow hopping, ballooning,etc.should be responded to immediately with the application of go around power which may require some mixture control adjustment for best power.( That Continental cough can come at the worst time.) Whenever practical, a smooth, flown on three point landing is going to offer the lowest touchdown speed. At high density altitudes, the true airspeed during the rollout can be significantly higher. Friction drag of tires ,wheel bearings, are significantly increased, while the flight control response is significantly decreased. The center of gravity is behind the main landing gear, which now has higher drag, which results in an airplane more likely to swap ends during extreme ground speeds while the tail feathers have the effective airspeed of zilch due to the thin air. I feel the directional benefits and lower groundspeed gained by the tailwheel firmly engaged with the ground outweigh any potential wing rise vulnerability due to the increased angle of attack. Every pilot should decide on base leg what method of landing is going to result in the smoothest, slowest landing which that pilot can best effect for the present landing conditions, then fly down and carry it out.

One final technique I employ during all landings. I make my first brake application early during the landing rollout. As soon as I feel I can safely slide my feet up to apply the brakes (tail on the ground, stick all the way back, rudder pedals momentarily stationary,) I make a moderate brake application. This accomplishes two things- first: it begins to slow the airplane down, killing some of the energy previously discussed. Second: I now have a feel for what I’ve got in braking response before I actually want (need?) it. One more benefit, if there were any type of brake malfunction to occur during this first application, I still have some speed and control effectiveness to deal with it better. Here’s to smooth (er) landings.

By |2016-11-13T09:33:19+00:00February 11th, 2013|Flying-Wire|Comments Off on Density Altitude Landings

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