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Owner's Corner

OWNER'S CORNER
Lloyd Stephens, Aircraft Owner and Board Member WVFC lgs@qnet.com  
 

Studies in Density Altitude Problems (Part 2)

The Mammoth Yosemite airport (MMH) is located about eight miles to the east of the town of Mammoth Lakes at an elevation of 7135 feet.  It has an east-west runway that is 7000 feet long.  Highway 395 runs parallel to the runway just to the south.  On the other side of the highway the mountains rise steeply to the crest of the Sierra.  Mammoth Mountain, which is clearly visible from the airport, sits at the head of the valley of the headwaters of the San Joaquin River.  This valley runs generally north-south and the terrain along the valley is much lower than that of the surrounding mountains. Frequently, when clouds form over the mountains, the lower terrain of the valley will be cloud free. Mammoth Mountain, at 11,030 feet, is the highest point between Mammoth and the central valley.  V-230, the federal airway that runs between the Friant VORTAC, 20 NM north of the Fresno International airport, and the Mina VORTAC in Nevada passes directly over the summit of Mammoth Mountain. 



As you look at the mountain from the Mammoth airport, Mammoth Pass is the low shoulder to the left of the mountain and Minaret Summit the shoulder to the right.  The low point in the ridges on both sides of the mountain is about 9,500 feet, and the terrain drops off considerably on both the north and the south side of these ridges.  Although the minimum IFR enroute altitude along V-230 in this area is 14,300’, you can fly VFR over the passes at a much lower altitude.  Because of this, the route over Mammoth Pass is used frequently by aircraft crossing the Sierra.

At about 4:15 p.m. on Wednesday, July 8, 2009, a Cessna 182P with two men aboard departed straight out from runway 27 at MMH and headed up toward Mammoth Pass.  The pilot (who was also an aircraft mechanic) and his friend were ferrying the aircraft to Oregon for the owner, who had just purchased it.  They had come from Phoenix and stopped for fuel at Mammoth before heading over the Sierra to Fresno, where they intended to spend the night.  There were gusty SW winds at the airport and the pilot had expressed some concern about these to the customer service manager at the FBO.  When they departed, she walked outside to watch them take off.  She thought it unusual that they headed straight out for the pass, because most planes circle to gain altitude before heading over the pass.  She also noted that the plane “didn’t seem to be gaining altitude very quickly.” 

The town of Mammoth Lakes is built more or less on a hill, so the town elevation varies from about 8,000 feet to about 8500 feet.  Just to the east of the town is the Snow Creek golf course.  Beyond the golf course, as you head toward the pass, the land becomes forested up to the lakes basin at about 9,000 feet, just to the east of the low point in the ridge that defines the pass.  The terrain is pretty open near the golf course but as you climb up toward the pass the mountains encroach on both sides, limiting the room available to turn around.  The plane continued the climb up into this area at a relatively low altitude.  It is likely that the plane encountered downdrafts as they neared the pass.  It must have become obvious to the pilot at some point that he was not going to make it over the pass but, by the time he realized this and tried to turn around, he had little room and very little altitude to do so.  As the airplane turned it lost altitude and, unable to recover, it descended into the trees.  A witness described the plane “coming up the draw” into the lakes basin, making a very low altitude turn, and then hitting trees.  The NTSB report indicates that there were multiple tree strikes starting about 1000 feet west of the main wreckage, which was located at an elevation of 8,575 feet.  There were no survivors. 

The NTSB report indicates that, at the time of the accident, the MMH AWOS reported winds at 240° at 14 knots gusting to 26 knots and a temperature of 24°C (75.2°F). No mechanical problems with the plane were found.  The density altitude at the accident site at that time was determined to be 11,300 feet.

It is difficult to tell what the pilot of this plane was thinking.  First of all, in this area, he had a choice of airports at which to obtain fuel, and MMH is perhaps not the best choice.  The Bishop airport would likely be a better choice.  Bishop is only 10 minutes flying time to the southeast of Mammoth and, if they had flown up the Owens Valley he would have passed over Bishop.  The fuel is considerably cheaper there, the airport elevation is only 4124 feet and, although there are high mountains on all sides, there are no high obstacles in the vicinity of the airport, giving you time to climb above the terrain even if it is hot.  Although we don’t know how much fuel he put in the plane, the weight of the fuel may have been a factor in reducing the climb capability of the aircraft (and it certainly would have been, if he had filled it up). 

Second, why did he choose not to circle to gain altitude like most planes do?  When you depart from runway 27, a 90° right turn takes you away from the mountains and out over fairly flat terrain that then drops off to a valley where you can climb safely to altitude before heading for the pass.  It is true also that the pass doesn’t look that high when viewed from the airport.  Eight miles away though, to fly through the pass at 10,000 feet would require a 375 foot per mile climb, easy to do at sea level, but much more difficult at altitude on a warm day.

Third, although he expressed some concern about the winds, the pilot didn’t seem to realize that it would likely be much windier with potentially significant downdrafts on the northeastern side of the pass with a strong southwest flow.  Wind flows like water, along the path of least resistance, in this case through the passes--up over the ridge and down the other side.  Given a plane flying anemically, add some downdrafts and a turn that probably caused a loss of lift, and the result was foreseeable.

As we discussed in the article last month, one of the most important things to realize about high density altitude is that it can seriously affect the airplane’s ability to climb.  It’s not just about getting the plane off the runway, it’s about clearing all obstacles once you are in the air.  Most of the accidents involving density altitude involve three distinct factors: HIGH, HOT, and HEAVY--a relatively high altitude airport, a relatively high temperature, and a relatively heavily loaded (or overloaded) plane.  Winds can also be a significant factor.  These are all things that need to be considered and managed in flying from a high altitude airport.   As we discussed also, most high altitude airports are located in mountainous areas, so obstacle clearance is a big issue.  You may not have a choice of which airport you are using, so let’s look at some of the other factors in this accident with a view toward how you can fly safely from these airports.

     At any altitude, the lighter the plane is loaded, the better it will climb.  Most light planes do not operate well at higher altitude airports when fully loaded, so you may want to limit the weight by reducing the number or weight of passengers, reducing the baggage, or reducing the amount of fuel carried.  Look at the graphs in the POH to determine not only the required runway length, but also the rate of climb given the plane’s weight and the temperature.

     This flight took off at 4:15 p.m. which, in July, is just about the hottest time of the day.  You will recall that density altitude is the field elevation corrected for temperature, i.e., the lower the temperature, the lower the density altitude.  In the mountains and the desert, the best time to fly is in the morning.  It’s cooler, and there is less turbulence and less wind.  If you can’t go in the morning, consider waiting for it to cool off in the early evening.  The temperature in mountain and high desert areas can vary significantly.  At Bishop it is not unusual in the summertime for the temperature to rise close to (or over) 100°F by mid afternoon, but then cool off into the high 40’s at night.

     Be aware of any obstacles, such as mountains or trees that you must clear and plan to cross them at an altitude that provides significant clearance, even if you have to circle to do so.  By doing so you maximize your options in case of downdrafts or other wind conditions.  Although I have seen pilots flying through Mammoth Pass only a few hundred feet above the ridge line, I usually do so at a minimum of 11,500’ and even higher if it is windy.  Mountain flying experts suggest that you cross ridge lines at a 45° angle so that you can turn away easily if you encounter downdrafts.  That may not be practical in cases such as Mammoth Pass.  Additional altitude gives you the ability to turn away more easily and the room to do so, or to continue ahead to fly out of the downdraft.

     Be aware of what the winds are doing, both on the surface and aloft.  Learn to visualize this and plan your flight accordingly.  Be aware that because of the slower rate of climb winds can have a greater effect on the plane.  Surface winds in the mountain areas generally are calm in the mornings and can pick up significantly in the afternoons.  Downdrafts and turbulence are to be expected on the downwind side of mountains and ridges, smooth air and updrafts on the upwind side.  When winds over Mammoth Mountain are from the northwest (as they frequently are after a frontal passage), flying over Minaret Summit, on the upwind side of Mammoth Mountain, is a better choice to avoid the more turbulent air over Mammoth Pass.

It is not my intention in looking at these density altitude problems to make you afraid to fly out of high altitude airports, but rather to make you aware of what you need to consider to fly safely from them.  I am not a flight instructor, but I have flown successfully out of both Mammoth and Bishop for a number of years in both a Piper Archer and a Piper Comanche, so I thought it might be useful to pass on a few tips to you. I hope this discussion will be helpful to you.

Fly safely!!

If you haven’t already seen these, you might also be interested in the following short videos depicting density altitude problems, each with a little instructional component, as well.  Note how, in both cases, the airplane doesn’t exhibit a classic stall but, when the pilot raises the nose to clear obstacles, instead of climbing, the airplane loses lift and descends.

http://www.youtube.com/watch?v=OVM3RRd1vf0

http://www.aopa.org/asf/epilot_acc/lax07fa258.html

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