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I recently received the December edition of EAA’s Sport Aviation magazine. As is my custom, I always check out Steve Krog’s “The Classic Instructor” commentary first. His article about density altitude brought back a memory of a long-ago flight.
As a 500-to-600-hour private pilot, I thought I had an understanding of density altitude and its effects on an aircraft’s performance. But that understanding changed on a hot, humid July day in northern Alabama!
In the mid 1990s, a buddy suggested that he, his wife, and I fly to Talladega, Alabama, in my Cessna 172, to attend the July NASCAR race. It sounded like a grand adventure, and I was all for the idea. Our flight from western New York was pretty uneventful until departing from our second fuel stop in northern Alabama. There, without much consideration, I did as I always had and told the fueler to top off the tanks. I was completely unaware of the lesson I would soon learn.
Even at low airport elevations, high temperatures can send DA uncomfortably high.
The subsequent takeoff began normally enough—I didn’t necessarily notice if we became airborne a little farther down the runway than normal or not. But once airborne, I slowly became aware that things weren’t going as expected. After liftoff, the climb rate of the 172 was downright anemic to say the least. It was clawing the air trying to climb, but without much success. We were barely climbing, but we weren’t skimming the rooftops either, and at least we weren’t descending.
I also noticed that the airspeed wasn’t increasing. In fact, to achieve whatever climb rate I did have, it was hovering somewhere just above stall. I was working hard, trying to coach the plane to climb by attempting to find the right pitch angle where the wings would produce the best lift, while hopefully at the same time maintaining an airspeed somewhere above stall. The goal was to slowly accelerate in order produce a better climb rate.
About now, there are readers that are wondering, how heavy were they? What was he thinking when he had the tanks topped off?
Obviously, I wasn’t thinking about performance or density altitude. It probably isn’t a newsflash that Alabama in July is hot and humid. But none of that information was considered as part of my planning because the airport elevation was relatively low, probably somewhere between 800 and 1500 feet.
In retrospect, I lacked a true understanding of the whole density altitude topic. I have decided that my lack of understanding stemmed from the fact that when density altitude was discussed during training, it was discussed as being an issue when high, hot, and humid.
Because all my flight training and flying up to this point had been in the east, I thought to myself, “Self, since I am flying in the East and density altitude presents a problem at high elevations (like if I was flying out West), then except for the written test. I don’t need to remember or apply any of this because it won’t affect me here where I fly.”
As I continued to build flying hours and I didn’t experience any issues with density altitude, it just reinforced that thought… until that day in July down in Alabama!
If I had thought about density altitude, here is generic example of what it may have been that day:
So, how/why did my thought processes take me down such an erroneous road?
For what it is worth, here is my take. When learning something new, or when our brains become overwhelmed with information, mine (and possibly yours) tends to “weed out” extraneous information. It decides what information is not important and what information it does consider important.
We then remember what was deemed important and the remaining extraneous information “gets shelved” in the back of our minds somewhere or is discarded completely.
As a result, we end up believing we have an understanding of a subject, procedure, or situation when we don’t. We usually aren’t even aware of the misunderstanding until we get that unexpected wakeup call. Hopefully, one that doesn’t end up resulting in a bad outcome.
As Steve Krog reflected in his article, “In order to protect ourselves from the effects of density altitude, we must first understand what it is and how it impacts flight.” This is true concerning all aspects of our flying. We must ensure that we first understand the subject, procedure, or situation. We must not allow ourselves to settle for what “we think we understand.”
As for that long-ago trip, it added to my experience, it is a great memory, and we had a great weekend. I am not sure my friends even realized that our takeoff was any different than any other. Talladega was as exciting and unpredictable as always, but boy was it low, hot, and humid during the race.
Editor’s Note: This article is from our series called “I Can’t Believe I Did That,” where pilots ‘fess up about mistakes they’ve made but lived to tell about. If you have a story to tell, email us at: [email protected].
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I learned a lot about density altitude flying a T-38. Sitting in the squadron one day, our Ops Desk called on the squawk box saying, “We’ve got a plane that needs two IPs to fly an out-and-back to get some hours on it before it goes into maintenance, any volunteers?” I looked at the other IP in the room and we headed for the door together. The only caveat was we get it back today. So, from Vance AFB (END) we decided to fly to Peterson AFB (COS) in Colorado Springs. It was a beautiful summer afternoon and I took the front seat for the first leg while the other IP got in the back. He was going to log some instrument time and approaches while I was the safety observer; we would switch roles on the return leg. Everything went smoothly as he flew a TACAN approach, followed by a PAR and then and ILS. We then got into the VFR pattern to fly some back-seat overhead touch-and-go’s. His first landing was nice and smooth, but, as he applied power, he said to me, “Something’s wrong, the airplane’s not responding.” We were about a third of the way down the 11,000 foot runway and I could tell we weren’t accelerating like we should have although all instruments indicated the engines were at 100%. I took control and selected the afterburners on our two J-85 engines. We then got airborne easily, but I too could tell the airplane was not performing as usual. I requested a closed to a full stop, which tower cleared us to do, and I made a normal landing. As I taxied into the transient ramp, the other IP called the weather shop. He then told me, “We’re in a Category III situation.” That meant the distance required to accelerate to single engine takeoff speed and then either abort the takeoff or continue to takeoff was greater than the runway length. Our regs prohibited us from taking off! Inside, we got on the phone and called back to Vance to tell them of our situation. After being read the riot act, we were told to stand by. We soon got a return phone call and were told a female pilot candidate at the Air Force Academy needed to be evaluated to see if she could work the controls in the T-38. Her short stature might prevent her from moving the rudder pedals through the full range of operation and the manual canopy might be tricky for her to close. She passed and we were ‘off the hook’. However, we were told, in no uncertain terms, that we had better plan for a VERY early departure the next morning before the field elevation (6187 feet) and morning temperature captured us again. Needless to say, we were up early and made it home safely that next morning. Lesson Learned!!!
I drove our company’s Merlin IIIC over to Bermuda Dunes a couple weeks ago and hung around the Crown Aero FBO for about three hours while awaiting the boss’s return. They’re great folks over there at Crown, by the way… Anyway, as I was sitting there exchanging useless, polite dribble with some of the locals, a quite effervescent (pardon the term) young fellow came in and sat down with a cool drink and joined the conversation. He soon let it be known that he had recently obtained his Private license and had also purchased one of those sleek new plastic amphibians that seem to be all the rage these days. He passed around some photos of it, but I don’t remember what he called it. (I can’t afford it anyway…) He then beamed about planning to take a couple of friends with him up to several lakes in and around the Sierra Nevadas soon – this coming summer in fact. After much oohing and aahing, someone in the room, I’ll not say who, finally got it in his mind to ask the young man what the horsepower was in this marvelous new machine of his. “One-hundred-twenty” was the deadpan reply. I was stunned. He said it dauntlessly, as if it warranted no further mention or discussion. Most of us in the room snuck quick glances at each other in turn with widened eyes and open mouths… Now, to make a long story short, I won’t go into describing and detailing all the scientific and philosophical debates about density altitude which quickly shot their way to the front of the “hangar-flyin’ class” that afternoon; however, needless to say, the young man soon departed for destinations unknown promising to reevaluate his thinking before heading up into the high range this summer. One-hundred-twenty horsepower? Really!!?? Flying around the Sierra Nevadas?? It’s not much more than a powered kite.
curious how u got down to stall speed from vx or vy
way behind curve arent u
too early rotation
too steep an initial climb
4000 foot density alt should b no prob in a 172 unless its unexpected
google failed me in finding an airport over 1000 feet in al
highest airport in nh is twin mountain 1549 with a 2600 ft runway but 105 deg and 95 hum not likely there (nor in al)
read the poh and perf tables and follow them
Hello Mr Gillett
Thanks for your comment
You stated: “curious how u got down to stall speed from vx or vy”
My reply: I guess no matter how well I tried to describe the situation there is always room for it to be interpreted differently. No where did I say “I was at Stall Speed” What I did try to say was; “I was working hard trying to coach the plane to climb, by attempting to find the right pitch angle where the wings would produce the best lift. While hopefully, at the same time maintaining an airspeed somewhere above Stall that would allow us to slowly accelerate in order produce a better climb rate.”
You stated: “way behind curve arent u”
My reply: Yes I was. Not only behind the Airplane due to its loss of efficiency. But, I was also way behind in my awareness of the overall situation that affected my decision making. (Which was the overall point of the story)
If I had taken into account the aircraft’s weight and the effects of Density Altitude I would have only fueled the tanks to half full and the take off would have been way less eventful
As for your checking on airport altitudes.
The flight took place in 1994/1995 and there have been a lot of sunrises and sunsets since then. Since I tend to suffer from CRS (Can’t Remember Shxx) I only made a generic statement concerning airport field elevation “the airport elevation
was relatively low, probably somewhere between 800 – 1500 feet.” Which is why I entered a generic guess for the Airport Altitude value of 950 feet in the illustration.
By the same token I have no idea which airport I landed at for the fuel stop. Though I did find a couple around the Georgia/Alabama Border that were 1125 feet, 1032feet. It’s true the remainder were not as high.
You Stated that: 105 deg and 95 hum not likely there (nor in al)
My Reply: Not sure where you saw the 105 degrees. I used 95 degrees and 95 for humidity (in the Generic Example) I have no way of knowing the temperature. But, sitting at the track I can tell you IT WAS DAMN HOT
So thanks for the feedback. I’ll strive to do better in describing a situation. Also, when I don’t have specific information, I’ll strive to make it clear when I am making broad statements.
I once took a co-worker of mine for a little site seeing jaunt. July, hot and humid, Cessna 182 RG. Since I am only 160, and him at or around 250, I didn’t give the weight and balance much if any thought. My bad. Got a lesson taught to me that day. Since said aircraft has the extended range fuel tanks, and they were full, I had around 500LBS. of gas. Took up a lot of real estate to get airborne. Climb performance was in the basement. Also took a long distance to land when we got back. I have not repeated that mistake. Could have been tragic had I flown a 172….
I became very aware of density altitude instructing in a 65 hp, J3 during the summer in Ohio. As 195lb adult I had to limit the weight of the student and amount of fuel. When fall came and the OAT dropped 30 degrees it was a different airplane. Not a jet but not as scary
My first cross country flight after getting my pilot’s license was in a rented PA28-140, with my 10 and 11 year old daughters on board. They were light, and I was not heavy yet, so full tanks were not a problem, even with the 2,000 foot runway. Field elevation was 680 MSL. Half an out later, I landed in a grass strip where I was to drop off the girls. When friends arrived to pick up the kids, they had brought neighbors with them. “Seán, take them for a little flight”, they asked unexpectedly. “Sure” said I without thinking. “Climb in”, and three adults joined me in the Cherokee, which still had about 44 gallons of fuel. It was a summer afternoon, and the runway was long enough for me to get airborne. But the plane did not want to climb, and I did not want to pull back on the stick. With the speed at only 72 mph, “Will I make it over the wires, or will I have to go under?” went through my head. I made it over, and did a very gentle climbing turn to the left, then downwind, base and final. It was a short flight, but I have cited it many times since as an example of being forward of c.g. at a high density altitude in the decades since. I may have been overweight also.
Great subject. The conditions of that takeoff probably reduced the rate of climb about 35%. If a pilot has never before seen a significantly shallower climb angle, the natural reaction is to try to make the airplane climb out at a familiar climb angle which of course is not going to work. I once flew at Leadville CO where our rate of climb varied between 100 and 300 at Vy. A descending breeze from the rising slope on the departure leg brought the rate of climb into a descent – we had to turn base to avoid a CFIT. Out of the pattern, when I started to transition to my usual cruise climb speed, the rate of climb immediately would go to zero. After that experience, I included some high density initial climbs with all of my students simulated by reducing the power to get a lower rate of climb.
I fly a Maule 4 seater 180. I took out the back bench seat, so it’s become your basic 2 seater. I would never take up 3 (or more) adults anyway, so it was of no use. Now, if I take another adult, I won’t fly with more than 35 of its 70 gallon useable fuel. Too much weight otherwise. One trip I took off solo on a wet dewy morning from a 2000′ turf runway, with almost full tanks. Bad idea. Barely made it out. Full tanks (70 gallons) + wet grass + high humidity = I’m lucky to write about it here.