Monday, May 26, 2008

Density Altitude

I'm not huge on brand loyalty. I usually go with the cheapest product when I'm at the grocery store but for airlines I'm beginning to notice the differences between carriers each time I fly. Jet Blue so far has impressed me the most. They have the newer jets, cleaner planes, better seats (eliminating a first or business class is brilliant for domestic flights) and great customer service while en route.

On my way out to Phoenix I was really pleased with the candid manner in which the pilot informed us passengers why the cabin was getting hot and why we might have to stop over in Denver or some other way point for refueling. My flight coincided with the headlines in all the major papers about how the airlines were trying to conserve fuel given the rise in oil prices.

One of the ideas was to slow the planes adding a few minutes onto the flight times but greatly reducing fuel consumption just as if you slow your car on the highway to 55 MPH you'll save fuel as opposed to driving around 70 MPH as I do. Another idea was to short load fuel so that the payload when taking off is reduced decreasing the fuel spent just to get the aircraft off the ground and up to cruising altitude.

This seems to have been the case with my aircraft when the pilot told us that the cabin temperature would be increasing as he reduced the load on the A/C in order to conserve fuel. We were only flying from New York's JFK to Phoenix's Sky Harbor but with the encounter of jet stream head winds double what had been fore casted (Nearly 150 MPH!) the plane was laboring to keep up her speed.

The Pilot was forced to decide whether to risk running out of fuel and stay in the calmer less turbulent jet stream or decrease altitude and fly in more bumpy conditions where the winds weren't as strong. He opted to try and make it without stopping for refueling and therefore turned down the A/C and turned on the seat belt light as we descended into more turbulent air.

I found this critical decision interesting because it reminded me so much of steaming south along the Pacific coast of Japan and encountering the Kuroshio current. Do you head further offshore to try and find a counter current to push you towards your next port, head for the coast to break free of the current and instead contend with heavy coast-wise traffic, or do you sacrifice time and keep it simple by steaming straight into it?

Though it's not likely that an opposing current would cause a ship to run out of bunkers at sea it is still the same decision process. I have entered ports where the engineers were about to switch over to distillate because the ship was sucking up the last of the heavy fuel oil because the company was short loading fuel either because of price differences from the departure port and the arrival port or just because they thought it imprudent to sail a ship with more fuel than necessary for the upcoming voyage. In that case a foul four knot current would have made the difference.

Yes, these are the things I think about when flying across the country and I have the customer conscientious pilot's at Jet Blue to thank for it. As an end note, while driving across country with my uncle, a hobby aviator I learned another very interesting piece of information, this one in no way comparable with maritime matters but interesting none the less.

My uncle had mentioned that while passing through Page Arizona we might rent a small aircraft and spend an hour or two flying over the Grand Canyon. Unfortunately due to the soaring temperatures we encountered in the desert on our way through Nevada, it was 109 Fahrenheit, he was having second thoughts. He explained the concept of "Density Altitude" to me so that I wouldn't be too disappointed.

Aircraft performance is rated at sea level and at a mild temperature providing ample air pressure for providing the necessary lift for flight. As air temperature increases though air density decreases causing a reduction in plane performance. To compound this if we had taken off in Page at an elevation of over 4000 ft than we would have had two compounding issues affecting our aircraft effectively reducing a ceiling altitude from say 18,000 feet for a Cessna 350 to much less.

The decrease in air pressure leads to a decrease in aircraft performance which increases the chance of stalling. Stalling occurs because the properties of lift have been negated by a change in air pressure over the wing due to perhaps banking the aircraft too quickly for the given Density Altitude. This causes gravity to take over and the plane to plummet which would have effectively put an abrupt end to our road trip as it has historically for flat land pilots from Illinois. For this and other reasons, such as my uncle having to get his corn crop in the ground for duck season this fall, we did not get to fly over the Grand Canyon.

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