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Less Delay At Lower Altitude?

Tue Sep 05, 2000 10:10 pm

An AP story in the San Francisco Chronicle for 23 August (page D7) starts:

"In a travel season rife with late arrivals and canceled flights, several airlines have begun flying at lower altitudes, trading fuel efficiency for on-time arrivals.
The FAA gave airlines approval more than a year ago to operate some short flights-- up to 500 miles-- at between 8,000 feet and 23,000 feet...."

The story says United sends 30-40 low-altitude flights out of O'Hare each day, "saving an average of two minutes on the ground and about 10 in the air, spokesman Joe Hopkins said." It also says AA will shortly start low flights (at Chicago?) and that NW flies them out of MSP and DTW. It also says what you'd expect: "But low flights are generally kept above 18,000 feet."

Can anyone begin to explain how flying at FL180 -FL230 will save ten minutes in the air? Can anyone give an example comparison?
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RE: Less Delay At Lower Altitude?

Tue Sep 05, 2000 11:55 pm

Well, the idea is that the airways in the medium altitude range are less congested and so would reduce delays due to traffic congestion in the higher alititude airways.

Why? Pilots/Airlines like to fly their planes in the higher altitude airways where drag is reduced (lower air density) and therefore require less fuel burn for the optimum cruise speed. Quite frequently, these popular alititudes are congested, and that results in takeoff delays or reroutings.


RE: Less Delay At Lower Altitude?

Wed Sep 06, 2000 12:23 am

You'd save time in the air at FL180 because you only need to climb to 18,000. A cruising AC is much faster than one that's climbing. Plus once you are at FL350, you need to get down. This means speed restrictions or vectoring in the decent around cruising traffic. As for saving time on the ground, there are often delays on major airways, example J80, however these only apply certain altitudes, at times, so you'd be able to eliminate that delay.

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RE: Less Delay At Lower Altitude?

Wed Sep 06, 2000 3:57 am

This "new" LADDR program is stuff that's been going on (informally) for years, but now it's an official program.

Precisely how much time going lower saves depends upon the winds.

Westbound, the lower altitude will give a higher true airspeed (TAS), and when combined with a lower (usually) headwind component (HW), the overall result is a higher groundspeed (GS) and thus shorter total ETE. For example, FL310 TAS 430, HW -50 = 380 GS.
FL240 TAS 460, HW -20 = 440 GS.

All that is at a cost in extra fuel for the lower, less efficient altitude.

Eastbound, it's often a different story. Going lower loses out on much of that jetstream-driven tailwind (TW), and the difference between the FL330 and FL250 groundspeeds isn't beneficial time-wise.

ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.
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RE: Less Delay At Lower Altitude?

Wed Sep 06, 2000 6:18 am

Primarily "hype" so it sounds like FAA is doing something about delays. Kinda like "free flight." As if enroute traffic is a problem?... Not!

Only works for flights leaving a hub. Flights inbound to ORD are usually sequenced for landing shortly after takeoff.
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