Depart With VNAV

A step-by-step guide for using GPS to monitor your climb gradient when terrain and obstacles threaten. (And even when they dont.)


by Paul Sanchez

As much as people love flying to the Colorado mountains for the skiing, fishing, hiking and grandeur, when all good things come to an end we unfortunately must depart an airport surrounded by the very mountains we love. If were trying to fly ourselves in instrument conditions, thats when things can go very badly.

As one example, on November 17, 1996, the pilot of a Piper Aerostar 601P decided he was going to go ahead and depart from Eagle/Vail, Colo. (EGE), despite poor weather. A corporate jets captain and first officer who had already decided not to depart, and in fact had off-loaded their passengers and baggage, encountered the Aerostars pilot and tried to convince him not to take off. The observed weather at EGE at that time included a 400-foot indefinite ceiling, one mile of visibility, light snow, light mist, temperature -2 C, a dew point of -4 C and calm winds.

Defining The Problem
In response, the Aerostar pilot explained to the jet jocks that he was RNAV equipped and had a handheld GPS, knew the area and could leave by feel. His clearance was to the Kremmling VORTAC, via the Gypsum Three departure procedure; climb and maintain 14,000 feet. He departed from Runway 7 at 1453 MST; less than twelve minutes later, by 1505 MST, the Aerostar had impacted the first of two ridges at 7681 feet MSL (EGEs field elevation is 6535 MSL).A pilot and four passengers were gone and a reasonably capable (for the terrain) airplane destroyed.

At the lower end of the performance/capability spectrum was a Piper Cherokee 180 whose pilot departed Runway 25 at EGE on August 1, 2002. Rather than executing a left turn to the 214-degree heading specified in the departure procedure, the pilot turned right to 296. The aircraft subsequently impacted terrain at 10,500 feet MSL. In this accident, two passengers were lost and the pilot and another passenger were injured.Calculated density altitude at EGE was 8834 feet; calculated density altitude at the accident site was 13,175 feet.

These are but two examples of failures to follow a charted departure procedure (DP) in mountainous terrain. There are many others, but the bottom line is that the mountainous airports of Colorado, Utah, New Mexico, Arizona and other states have a well-deserved reputation for being intolerant of poor technique and any failures to follow a charted DP.

One of the ways pilots routinely find themselves getting into trouble in these situations is to ignore comparing their climb gradient to the terrain gradient. The results are not very good for pilots or passengers and certainly not good for the publics perception of small airplanes.

In the above two examples there is a published procedure for departing Runways 7 and 25 at EGE, the Gypsum Three Departure (Obstacle). Among its other features, this DP specifies a climb gradient of 480 feet/nm until reaching 11,800 feet MSL when taking off from Runway 7. When departing Runway 25, a much steeper, 750 feet/nm climb gradient is required to 10,500 feet MSL. Despite the inability of most piston-powered airplanes to sustain the required climb rates-at a groundspeed of 120 knots, the Runway 25 DP requires a minimum climb rate of 1500 fpm to 10,500 feet-another operational issue is the mental math necessary to obtain an accurate howgozit while flying the DP in IMC. To put a finer point on the problem, how good is the average pilot at trying to recall the arcsine of 7.09 for Runway 25 or 4.53 for Runway 7? If we were that good at math, our SAT scores would have reflected it and wed be working for NASA. Or Burt Rutan.

Garmin To The Rescue
Fortunately, many pilots have something that does the required trigonometry faster than we can fail it: a Garmin 400/500-series navigator with its built-in VNAV function. In the first two of my articles in this series on using the Garmin 430/530 (February and April 2004 issues of Aviation Safety), we talked about using the units VNAV capability to descend to a specified height above and distance from an airport. And that capability is fine and dandy for a 3 descent profile to the airport elevation, presuming there are no obstacles, traffic or ATC-related issues.

But lets say were vastly more concerned about climbing out of EGE-or some other airport-and do not want to be found during the next spring thaw.The trick is to program the VNAV function on the Garmin 400/500 series so that it calculates the vertical speed required even before we leave the ground and displays constantly updated information on whether we are achieving the climb gradient required. This would be done by configuring the data field displayed below the NAV frequencies to show Vertical Speed Required, or VSR. Doing so will provide us a numerical value, expressed in feet per minute, which we can use to decide if were achieving the required performance.

Programming a VNAV waypoint is always limited to the waypoints that have not yet been reached in your active flight plan. So the first thing we need to do is create a flight plan. In this case, our ultimate destination is the Denver (Colo.) International Airport (DEN), only 105 nm away as the crow flies. Of course, the crow is slower and smarter than we are and can climb to its cruising altitude without needing a DP, so we need to do some knob-twisting while we are still on the ground. The sidebars below and on the next page will get you through the programming needed to set up all of this.

Flying It
After takeoff, if our current climb rate is greater than the vertical speed required, our climb gradient is higher than what is required. This will be displayed by the VSR value decreasing on a second-by-second basis. On the other hand, if our climb rate is less than the vertical speed required, the value displayed will be increasing and we are going to be in a heap of trouble almost as tall as the terrain we are going to hit in less than 5.3 nm. If you find yourself in this situation, our strong recommendation would be to make at least one 360 turn (although this may be problematic in IMC).

Luckily, the airplane were flying today has enough performance that the VSR display is decreasing. Once we pass the VNAV waypoint we created, the VSR blanks out, awaiting a new VNAV configuration, and were on our way.

A 750 feet/nm climb is unattainable for most non-turbocharged piston aircraft. At Eagle, one alternative is to use Runway 7 which requires a lower rate of climb. So, before even landing at one of these airports, every thinking pilot should calculate the aircrafts climb gradient. It does not do much good to land at Eagle, enjoy the skiing and, when ready to leave, realize that your aircraft could not do 480 feet/nm even at 5000 feet density altitude.

Also With This Article
“The Gypsum Three DP”
“Creating Flight Plans In Garmins 430/530”
“Programming And Flying The Gypsum Three”

-Paul Sanchez ([email protected]) is a CFII-MEI who does insurance qualification courses and avionics training nationwide at his clients locations.


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