Low Flying Aircraft at cruise alt - any possible reasons?


#1

While I was outside Wednesday afternoon putting up holiday lights, I noticed an aircraft that was at cruising altitude (i.e. not an aircraft landing or taking off from my local airport) and it appeared to be much closer than your typical aircraft at fly-by altitude, such that you could see more detail of the aircraft than normal.

I pulled up flightaware and found out it was a Mesa Airlines CRJ flight from oklahoma city to los angeles lax and it was indeed flying at its planned maximum altitude of 24,000 feet at about 455 knots. Looking over the last few days of flights, most flights occurred 34,000 to 38,000 feet and took 10-15 minutes longer to fly than today’s flight.

It got me to wonder why that aircraft was flying so low for a cruising altitude when I notice most cruising altitudes of 34,000 to 40,000 feet as planes fly high over the city.

I assume if there was depressurization at takeoff, the plane would return to oklahoma city or at least not fly at 24,000 feet but at some lower altitude where oxygen is more plentiful while getting ready to land at an emergency airport. Other thought was maybe engine performance would be better at lower altitude where there is slightly more oxygen available to use in the combustion process. I’ve ruled out avoiding the jetstream reason as it doesn’t appear there was a jetstream at 300mb or 250mb that would cause headwinds.

Any ideas or common reasons why an aircraft planned altitude might be that low?


10000 ft cruise altitude?
#2

Good question, and here are a couple of points.

first, anytime you see a jet flying for a long period of time at 24,000 feet (or 23,000 going east) it is probably because the regulations require both crewmembers flying a turbine powered airplane above 25,000 ft. to either: have available uncomfortable quick donning oxygen masks or, one crewmember to continuously wear their mask. The second pilot must also have a mask available whether it is one of the new quick donning masks or an older really uncomfortable mask. Since jets don’t normally carry extra masks a good bet is one or both masks failed the before takeoff check. (FAR 121.333 paraphrased) Uncomfortable is not in the regulation!!

second, turbine engines actually burn less fuel the higher they go. It is in direct proportion to the density of the air. By the time you get to 18,000 ft. the air density is half compared to sea level so, for the same power setting, a jet or turboprop has cut it’s fuel flow in half. So why don’t they fly at 90,000 feet? The reducing air density as you climb also affects the wing, it produces less lift the higher it gets. Eventually the gains from reduced fuel flow are eaten up by the less effective wing. Indicated fuel flow is a good indication of the power being produced. Half the fuel flow = half the thrust.


#3

Another very likely reason is that the airplane had a mechanical problem. As an example, most jets have two air conditioning packs, which supply air to the cabin for pressurization. If one of the packs is inoperative, the jet can still fly, and can maintain its pressure without any problem, but just as a precaution, it is required to only fly at an altitude below 25,000 feet, where the pressure difference between inside and outside is less than if the airplane were up at 35,000 feet. It is all perfectly safe for the passengers and crew, and the penalty to the airline is that the airplane burns more fuel at the lower altitude. But from an airline point of view, burning a little more fuel is preferable to canceling the flight. The malfunctioning air conditioning pack would then be fixed at the next overnight station, or when it is routed through a maintenance station.


#4

I’ve had to do that a couple of times in an ERJ.