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Owner's Corner

Lloyd Stephens, Aircraft Owner WVFC  

Studies in Density Altitude Problems (part 1)

It was a warm, but not overly hot, afternoon in early summer when the pilot of a Piper Saratoga and his three passengers departed from Runway 10 at Truckee-Tahoe airport for the flight back to San Jose.  The Saratoga was a fixed gear model with a 300 hp Lycoming IO-540 engine, so plenty of power.  The field elevation at the Truckee airport is 5900 feet MSL and Runway 10 is 7000 feet long.   The pilot had flown into Truckee a number of times previously in the Saratoga, mostly to go skiing, and felt pretty comfortable that he would be able to get the plane off the runway within that distance.

Runway 10 points more or less toward Lake Tahoe.  There is an overrun at the end of the runway and, within a short distance thereafter, Martis Creek Road angles across the departure corridor.  The terrain the other side of the road drops down slightly to a treeless meadow that extends for about a mile and then becomes heavily forested as the terrain rises to a high ridge before dropping off toward the lake.  The pilot had selected Runway 10 for the departure because it is the preferred no wind runway for noise abatement reasons, there was only a little wind, and because the meadow provided an opportunity to climb to a safe altitude before encountering the trees.

The pilot expected that the take-off roll would be fairly long, so he wasn’t surprised that the plane wasn’t off the ground by about halfway down the runway, but what happened when he rotated and the plane did leave the ground was entirely unexpected.  The plane climbed to about 15 feet above the ground and would go no higher.  He didn’t have enough runway left at that point to put the plane back on the ground, so he kept pulling back on the controls, trying to get the plane to go higher, with no effect.  As the plane went over the end of the runway, then the road, and finally dropped down over the meadow at about 15 feet, he realized that he had to lower the nose to gain more speed, but mentally that was a very difficult thing for him to do at such a low altitude.  By lowering the nose though, he finally gained enough speed to get out of ground effect and, only a few feet above the trees at the other end of the meadow, he made a shallow turn back toward the airport and they were on their way to San Jose, in one piece and considerably wiser. 

He rather sheepishly related this story to me and said that when this incident happened it really scared him.  As he said: “the pucker factor was extremely high.”  He said he realized later, when he had time to think about it, that he had thought at the time that, because the plane had used so much runway, it should be ready to fly, and pulled it off before it had gained enough speed to fly out of ground effect.  And then he had made the situation worse by keeping the nose up and trying to climb instead of lowering it to gain more speed.  He said that the fact that most of his other flights to Truckee had been for skiing had caused him to feel that the plane should be flying at the point when he rotated, even though the plane apparently didn’t think so.

The concept of density altitude is pretty simple, but it is sometimes hard to grasp.  Basically, it is the altitude the airplane “thinks” it’s at.  With reference to airports, density altitude is the airport elevation corrected for temperature.  The higher the temperature, the higher your plane thinks the airport is.  This doesn’t make much difference to you at low altitude airports (if the airport is at sea level and it’s 104° F, the DA is only 2816 feet), but it can make a lot of difference at higher altitude airports.  I don’t know what the actual temperature was in the above scenario but, assuming the temperature at Truckee was 80-85° F, the DA would have been between about 8500-8900 feet.  That means that the plane “thought” it was taking off from a field that was almost 9000 feet high, not one that was at 5900 feet.  Many aircraft take off performance charts don’t even go that high.  On the other hand, in the wintertime, assuming a temperature of 32° F, the DA would be about 5500 feet--quite a difference.  It’s no wonder that the pilot in this case thought he should be off the runway sooner.

Most folks seem to think of density altitude as affecting only the take off from an airport.  That is, if you can get it off of the ground while you are still on the runway, you’re OK.  Nothing could be farther from the truth.  The most dangerous thing about density altitude is that it can considerably reduce your ability to climb.  If you expect the same climb rate that you would get at lower altitude airports, or even at higher altitude ones, and rotate to the attitude that you would under those circumstances, you may be surprised, as this pilot was, that the plane will not respond the same way.

This paragraph is going to get a bit technical and you can skip it if you want, but for those of you who want to follow along, let’s look at the performance charts for the Saratoga as applied to this situation.  The airplane flight manual for the Saratoga indicates that the maximum take-off weight is 3600 lbs.  I don’t know what the actual loading of this aircraft was, so just for purposes of discussion I am going to make some relatively conservative assumptions about the loading.  The empty weight of this particular plane was 2197.5 lbs.   Assuming 4 persons on board (at 170 lbs. each), only 100 lbs. of baggage (maximum is 200), and 80 gallons of fuel (maximum is 102 gals.), that comes to 1260 pounds.  Added up, that’s 3457.5 lbs.  Let’s assume also that the temperature was 82.4° F (28° C).  This temperature is off of the Normal Procedure Takeoff Ground Roll chart, but we can extrapolate the 6000 foot line to that temperature and weight and we get a no wind ground roll of about 4800 feet, with a takeoff speed of 79 knots.  That figure includes full throttle before brake release, and is based on a three blade prop.  (With a two blade prop the distance would be more like 5500 feet.)  This distance could be shortened to about 3800 feet with 25° of flaps and a takeoff speed of 64 knots.  In either case, the climb rate after takeoff would be about 500 ft/min, just about half of the sea level climb rate.

Considering that the Saratoga is a fairly high performance aircraft, if you are flying a lower performance aircraft the climb performance will be even less.  In my Archer, at maximum gross weight under similar circumstances, the climb rate would be more like 300 ft/min.  Most high altitude airports are located in mountainous areas and this low climb rate in warm weather can create considerable problems.  We’ll discuss this issue more in next month’s newsletter.

So we can take away several things from this incident:

     At a high altitude airport when it is warm and the density altitude is high you should expect a longer takeoff distance than usual.  To minimize that distance it is important not only to lean the fuel mixture, but also to consider the configuration of the aircraft--such as use of partial flaps (as recommended in the Pilot’s Operating Handbook), and using full throttle before brake release.  Taking off into the wind can also considerably reduce the takeoff distance.  (It is rarely advisable to take off downwind at a high altitude airport, even if that is the direction you want to go.)

     Don’t try to pull the plane off of the runway before it is ready to fly.  Although the plane will actually be going faster than usual, the indicated airspeed you will use to determine rotation will be the same at altitude as at sea level.  Unless you have obstacles to clear at the end of the runway, gaining a little extra airspeed won’t hurt.

     Don’t over-rotate.  Since the rate of climb will not be as great as you are used to, you won’t be raising the nose as high as you are used to either.  So be cautious as you rotate.  Climb based on your indicated airspeed, not nose angle.  Remember that your best rate of climb (Vy) will decrease at higher altitudes and your best angle of climb (Vx) will increase.