The point of flying, however, is to get from Point A to Point B through the air. Under high density altitude conditions and in the vicinity of rapidly rising terrain, we need only modify our plan somewhat. While "flatlander" pilots may always think about taking off and going straight from Point A to Point B, those who fly in and around higher terrain will look at getting around things in their way.
For example, consider a departure to the west from El Paso International Airport in Texas, with an elevation of 3,958 feet, following V-16 on a day when ATIS reports a temperature of 27 degrees Celsius. Given our takeoff and climb performance, departing from Runway 26L and heading directly along V-16 in a Piper Arrow simply doesn't give us enough room to clear the ridge, especially with numerous towers located on the ridgeline.
In the example, avoiding the mountains is a simple football maneuver: Do an "end run." Note that there is lower-lying terrain to the south of the ridgeline. Although the flight path in this example will take you in close proximity to the Mexican border - which you cannot cross without air traffic control's permission - the border is easily distinguished by the Rio Grande River that separates the two countries. Staying between the ridge and the river allows us to remain clear of terrain without taking us seriously off course.
Let's look at another example: In Reserve, New Mexico, at 6,360 feet above mean sea level, doing an end run to clear the terrain when departing in most directions isn't a very good option. Because the terrain rises rapidly in almost every direction, something a little different is required, and it's almost as simple as an end run. Here, the option is to climb while circling the airport until you've gained sufficient altitude to clear the surrounding terrain.
What about terrain en route? Before taking a short cross-country flight from El Paso International to Sierra Blanca Regional Airport in Ruidoso, New Mexico (an 8,101-foot runway at 6,814 feet msl), we consult a weather briefer and find out that the current temperature and pressure at Sierra Blanca are 23 degrees C and 30.29 inches, respectively. Using a quick approximation for pressure correction of one inch pressure difference per 1,000 feet and 1,000 feet of density altitude for each 10 degrees C above standard, we arrive at an approximate density altitude of 8,500 feet. Already we have a slight problem: Takeoff performance charts for our Piper Arrow III stop at a density altitude of 7,000 feet. How can we determine the takeoff roll?
There are essentially two ways to get a takeoff roll estimate when the density altitude exceeds the performance charts in your airplane's approved flight manual or pilot's operating handbook. One method involves extrapolating the charts themselves. You should note that the aircraft performance is generally not linear with regard to density altitude; in other words, the mathematical plot or takeoff roll versus density altitude doesn't form a straight line. Therefore, simply drawing a straight line to extend the performance tables will, at best, give you what I like to call a "hand-waving" figure to work with. Perhaps the best way to describe the figure you arrive at is by calling it an "informed guess." You should keep that in mind when making a go/no-go decision based upon this extrapolation - especially if your extrapolation takes you far off the chart.
The second way to arrive at a ballpark figure for your takeoff roll is through the use of a Koch chart. With the Koch chart, you draw a line connecting the pressure altitude of the airport (which you can either estimate using the pressure lapse rate of one inch per 1,000 feet of altitude or determine directly by setting the altimeter to 29.92 inches Hg and reading the altitude off the instrument) and the temperature at the airport. From the center scale, you determine the percent increase in takeoff roll and the percent decrease in climb performance for the airplane. Note that you will need to determine the sea level takeoff roll under standard conditions in order to make this calculation, which you can find in your AFM or POH.
It's a foolhardy pilot who calculates an estimation and then attempts to take off from a runway that barely exceeds the arrived-at figure. You should give yourself a healthy margin of error and then watch the airplane's performance throughout the takeoff attempt. I like to have available at least 1.5 times the runway length arrived at in the calculator, and I prefer twice that length if it's an airport with which I am not already familiar.
Let's try the calculations, and see what we arrive at with each. Extrapolating the table, we arrive at a takeoff roll using a short-field technique of about 3,000 feet. Using the Koch chart and a sea level ground roll under standard conditions of 1,000 feet (obtained from the same performance table), we arrive at a figure of 2,800 feet (180 percent of the sea level figure plus the sea level figure) for the ground roll under the same conditions. Notice there isn't much difference between these two figures. Being conservative, I'd choose the table extrapolation estimate of 3,000 feet and multiply by two for my preferred runway length, resulting in a requirement for 6,000 feet of runway to attempt the takeoff. Sierra Blanca has an 8,101-foot runway, making this acceptable to me for the attempt. I'll do other things to enhance my airplane's performance if I can, such as departing at less than maximum gross takeoff weight and taking off into the wind and downhill if the airport has a sloped runway. After all, my book's figures were determined with a new airplane by a test pilot, and my plane is not new - nor am I a test pilot.
Even taking these precautions, I'll be watching the aircraft's progress closely. The first thing I'll want to see is that the airplane becomes airborne in the estimated distance or within a short distance beyond that. If it doesn't, my planned action is to abort the takeoff and stop on the remaining runway - hence my preference for plenty of runway just in case. Also, I want to see the airplane climb out of ground effect fairly quickly. Again, if it doesn't, an abort is indicated - meaning I'll be landing on the runway ahead. Remember to check your POH for any special leaning procedures specified for high-density-altitude takeoffs.
With these precautions in mind, I am fairly well assured that my trip to Ruidoso will be safe with regard to the amount of runway required. It's time to consider other factors.
What about the route of flight itself? It turns out that this route is more complicated than it may have seemed at first.
The first things I will have to take into consideration are the large restricted areas to the north of El Paso International. According to the chart legend, many of these areas are in continuous or nearly continuous use, so the best plan will be to avoid them. A narrow stretch of airspace (which local flyers call "the corridor") excluded from these areas will have to be navigated until just north of the town of Orogrande, and a straight-line course will not be possible. Farther along, I will also encounter the Beak Military Operations Areas, and although I can legally fly through them, I may want to minimize the time spent within them - especially if they're "hot." I'll be asking the flight service specialist specifically about these areas to determine how critical a factor they may be to my flight.
Restricted areas and MOAs are the rule rather than exception in the desert Southwest, so careful attention to aeronautical charts is a must to avoid penetrating these areas inadvertently or, in the case of MOAs, when activity within them might create more of a hazard to your flight than you might want to accept.
The second consideration I'll be looking at is the high terrain to the west of Sierra Blanca Regional. I can avoid some of this terrain if, instead of going up "the corridor," I plan the flight to take me over Pinon VOR and then east of the terrain to Sierra Blanca Regional, but this route will put me inside the Beak MOAs longer than I might like. Instead, I'll be looking for some other way of avoiding the terrain.
Here, what man has done on the ground is often a good hint as to what man should do in the air. In this case, checking the charts shows me a prominent highway that winds up through the mountains and passes near Sierra Blanca Regional. Roads typically follow the lower terrain, so this looks like a good way to avoid the bigger peaks and make it to my destination. Again, a straight-line course will not be appropriate.
Most pilots aren't used to flying this sort of winding route. We like to take off and go straight from Point A to Point B. But in this case, following the road has not only the advantage of providing a good way to avoid the high terrain, but also potential emergency landing areas should we encounter a problem along the way. We need not follow every bend and turn, but staying within gliding distance of the road may give us our best chance for a successful off-airport landing.
Careful attention to terrain features and the ability of the aircraft to climb are essential to avoiding problems in any mountain flying situation. Terrain can outclimb the airplane, leaving you with no way out. What may look like a pass can turn out to be a box canyon, trapping you into an accident. And in some areas, density altitude can cause even the low terrain to be higher than your aircraft's ability to fly. The best move is to obtain and study sectional charts for your route, get a thorough flight briefing, talk to local pilots, and plan the route that gives you the most number of "outs" should things not go as predicted.
Given that our flight won't follow established airways, how do we file it? First, filing should not be a question: We will file! For one thing, the terrain will not only make it harder to find us should we go down, but also make it harder for rescuers to get to us. It also will make it harder for us to obtain VFR traffic advisories from air traffic control, especially if our airplane's capabilities will require that we stay low - the terrain blocks radar as well.
When you are in an area where such coverage is possible, by all means request traffic advisories. Just be aware that in many mountainous areas, this will require that you fly at a fairly high altitude in order to be seen on radar, and your airplane may not be capable of the altitude required.
Even with a flight plan filed, a search for a downed aircraft won't be started until that aircraft is 30 minutes overdue at its destination if the flight plan remains open. Is there another way that we can cut down the search time should we have to land off airport? Yes, through a process called position reporting. Position reporting involves making periodic calls to a flight service station and reporting the progress of your flight. On our flight, we may want to call when over the town of Orogrande, over Alamagordo, and over the racetrack at Mes-calero. You can inform the flight service specialists of your location and when and where you expect to call again. Lack of a call when expected will alert the FSS that there may be a problem. If you should subsequently become overdue, the transmissions you made along the way can be used to narrow the search and get rescuers to you sooner.
When it comes to filing a flight plan, flight service stations in mountainous areas are accustomed to receiving flight plans that might sound unusual anywhere else. In this instance we'll be following U.S. 54 north to the city of Alamagordo, then U.S. 82 to Sierra Blanca Regional. If you call to file your flight plan, you'll find that the briefer on the other end of the line will accept this description of your route of flight without question, as such descriptions are common in mountainous areas. If you file by DUATS, you may have to define some segments in the form of either going from one latitude and longitude to another, or one VOR radial and DME fix to another.
Generally, it's easier to simply call a briefer to file your VFR flight plan. This has the added advantage of allowing you to get any last-minute weather developments along your route at the same time.
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