Happy Friday, everyone.

I read this on the NBC News website this morning, and thought I would post it here for comment:

"Commercial airlines are keeping tabs on mercury levels for any indicators that sultry conditions could throw flights off course, according to the AP."

Does anyone know what this means?

 

It's likely a thinly-veiled reference to density altitude.

 

That is oddly written.

'Mercury levels' here really means temperatures. High air temps reduce lift generated by a wing; they also reduce thrust (air is less dense). It means that an aircraft that has the range to fly xxx-yyy with a full passenger load at 80 degrees F from a given runway length may lack the range to do so at 110 degrees F. Offload passengers (tough when load factors are typically >80%), cargo, or offload fuel and make a fuel stop on the way.

Range charts for aircraft are based on a 'standard day' and that is actually a pretty cool air temp - more like Seattle in May than Phoenix in July.

 

Here was the sentence after the one posted:

A wave of triple-digit heat forced several airlines to bring operations to a halt after Phoenix climbed to 122 degrees in June 1990.

This is mostly for the Desert SW. Many areas will be approaching 115-120 degrees (LAS, PHX, PSP).

 

Density altitude ("DA" in pilot lingo) affects many things, but the factor that is usually most critical is an aircraft's ability to get off a runway and climb. If it's very hot out, that can require a much longer takeoff roll to get off the ground, and dramatically reduced climb performance.

If the runway in use is short, or if there's high terrain (mountains) close-in around an airport, density altitude can be extremely dangerous due to degraded aircraft performance.

DA is actually one of the leading causes of fatal aircraft accidents in the US west, but those accidents almost always involve smaller airplanes that are more marginally powered. Commercial jets typically have plenty of performance to overcome DA, but if conditions are right (higher altitude, high temperatures, shorter runways, full loads) it can make enough difference to be dangerous.

 

This is more apropos for the TravelBuzz forum so we're moving it there.

Thanks.....

______________________

Cholula
Alaska Airlines Mileage Plan Co-Moderator

 

Heat waves would occasionally impact my regular DCA-MCI flights back in the '90's. US Airways 737, flying almost to the edge of the DCA perimeter at the time.

It usually took both high heat in DC plus some weather along the flight path to KC, but about once a summer my Friday afternoon flight would take on a light fuel load and then make a pit stop along the way.

To this day, this is the only reason I've ever been to SDF.

 

I recall this being an issue a couple of times when I was flying out of Denver - in the old Stapleton days.

They needed volunteers to give up their seats as the heat and high altitude meant less lift (is that right?) and more fuel used on take-off and therefore, less range?

 

it is 113 F(45 C) at my house in Tucson right now. That's just not right! This must be the reason that they refer to air-conditioning around here as "refrigeration."

 

Heat as well as altitude can affect the lift. Hence the reason when they built the new DEN airport they took all that into consideration and then some. With their 16000 foot runway Id imagine they can take off if its 120. You would also find that most airports in the mideast, Africa, and other places where it gets well above 100 quite often also have 14 or 15000 foot runways. Not sure why they didnt plan for that at places like PHX

 

I was in Phoenix that day. They said the airlines didn't have the existing manuals for taking off at that temperature. Airport was closed for about 30 minutes.

We cooked an egg on the sidewalk. Didn't eat it though

 

flight crews on older jets like the 727 that don't have digital flight decks have to use the paper WAT (Weight/Altitude/Temp) charts in the flight manuals to determine takeoff distances and speeds ... the WAT tops out at 120F, and while it was legal for the crew to interpolate between lower temps, it was not legal to extrapolate beyond the printed limit

DL 1797, PHX-LAX, 27 Jun 1990

 

When I lived in Phoenix I was told if the temp gets above 120F there's a good chance they'll close the airport. It did hit 120 once but I didn't hear if they closed it or not.

 

Yes, as Cyclo reported above, that day it reached 120 in Phoenix the airport was closed for a while. I believe that's the upper operating limits on commercial aircraft. I lived in PHX at the time, but was at my company's HQ in Silicon Valley that day. I remember phoning my family and telling them, "Don't go outside."

 

1990 was a crazy weather year in Phoenix. On Christmas Eve, it snowed at my house. Low temperature that night was 26.