Thursday, 29 August 2013

Guest post from “D”

A bit of an intro first. I think D and I found each other through someone else’s blog. We quickly discovered that we flew from the same airport and had many things in common. We meet occasionally for some “in person” hanger talk and exchange frequent emails. D has many interesting tales. Recently he went off to do something rather cool and I asked him if he’d be interested in a guest post.

In his own words “I sat down at a keyboard, and this is what came out….”
I'm a Canadian pilot who did all of his training at WMAP's airport in the Great Lakes. I'm used to flat horizons, long-range radio reception, and climbing with the mixture full rich.

In mid-August I attended a technical conference in Logan, Utah and had the opportunity to do some very different flying. Logan (KLGU) is a pleasant little airport in Cache Valley, Utah. Its main strip is 9,000' long, at 4,475' above sea level. It is a "no-tower" (i.e. uncontrolled) airport, despite the physical presence of two towers.

I knew that the procedure for joining the circuit was different in the USA than in Canada. Americans are supposed to join mid-downwind, at 45 degrees. It seemed, however, that I was the only one doing this. Other pilots using the airport would join however was most convenient, including long straight-in approaches.

The biggest challenge for a sea-level pilot is the density altitude. Logan isn't just high, it's hot. In the afternoon it might be +30C, and a 4,500' pressure altitude turns into a 7,000' density altitude. The mountains that delineate Cache Valley rise to 9500' ASL or more, with density altitude 12,000'. The service ceiling of a C172N is listed as 14,200', letting us clear the ridges with 2,200' -- just enough for comfort.

My first adjustment was in the use of the mixture control. At sea level the mixture is run full-rich while climbing and descending, and after an engine check (prior to landing, stalls, etc.). The mixture is leaned only during cruise, and there it is set to 25-50 RPM lean of peak. In the mountains, the mixture is adjusted to one turn rich of peak at all times. This is done prior to takeoff as part of the runup, to ensure maximum takeoff power. During the climb it is necessary to continually adjust. Using the correct mixture setting is good practice during the landing approach so that maximum power is immediately available in the event of a go-around.

I have always found mixture setting via the tachometer to be an imprecise science. I worked mostly by ear, leaning until the engine sounds unhappy and then enriching a turn or so. Long descents at low power involved a series of mixture nudges every 1000' or so. The decreased aircraft performance also takes some getting used to. At 12,000' density altitude the engine's maximum horsepower is 60% of the sea-level value. The throttle spends most of its time at or near the firewall. Even so, there is almost no danger of overspeeding the propeller in straight-and-level flight. The engine just doesn't have the oomph.

On my initial check-ride the instructor asked for a full-power stall. This worried me, until I realized just how little power it was. On my second flight I had three adults in the aircraft and was unable to climb above 9,500' -- at full throttle and Vy the altimeter just would not move. For my third flight I summoned up the courage to make a run over the mountains. With just one adult passenger and a half-load of fuel we attempted to 9,250' pass leading to Bear Lake, Idaho. At these altitudes hypoxia begins to be a concern.

Thankfully humans are mammals, with constant temperature and humidity lungs, and so we are only affected by pressure altitude and not density altitude. Even so, the law limits the crew to no more than 30 minutes above 10,000', and not to exceed 13,000', unless supplemental oxygen is available. As we climbed towards the pass and reached 10,000' the stop-watch was started, and we knew we had only half an hour to get down into the valley on the far side. As it happened we reached a peak altitude of 11,700' (my new personal record) and cleared the ridge without incident.

As always, it was illuminating to talk to my fellow pilots at the FBO. While I was awed by the possibilities and dangers of mountain flying, many of these folks expressed wonder and concern about low altitudes. Some wanted to take trips to sea-level, just to see what peak aircraft performance is really like. I'm very glad to have flown in both regimes, and look forward to getting back into the mountains next summer.


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