There was a recent trip report that involved a flight that was over-weight due to the recent heat wave we had. Does anybody know how temperature affect the takeoff weight limit? Just curious. Thanks.

When the temperature goes up the air becomes less dense. Less dense air produces less lift. For a given loading of the airplane it will take more runway to get off the ground.

seawolf: When air gets hotter it expands and becomes less dense, the result is less lift at the same airspeed. It is not uncommon when there are higher then normal temperatures for there to be weight restrictions on aircraft. I can rember a number of years ago flying DEN-EWR on a flight that normally would have been a non-stop having to refuel in MCI due to temperatures of around 100F in Denver.

This has caused the airlines to ask for volunteers to leave when I was departing Phoenix a few times. Once it was 128 degrees F.

back in the late 80's, when PHX & DFW were getting socked with 115 degree weather, I was told all flights into & out of PHX had been cancelled until 8pm, due to heat. Airport just plain shut down...no lift!

I've left Phoenix at 116 before. The pilot said that planes could not maintain lift at 122 degrees farenheit. Not sure if this is the right temp though.

Teperature and ALTITUDE play a huge roll in the take-off distance needed for a plane. Both factors affect air density. This is what is referred to by pilots as "hot & high" performance. Taking off from Aspen in the middle of August often cripples a plane's range...

Let me distill a number of relevant things others said into one ordered post.

The amount of lift a plane can generate at any speed is dependant on the "density altitude". That is a calculated altitude that depends, among other things, on temperature and altitude. When the density altitude increases, as it does on a hot day, a plane will require more runway to accelerate to a speed that produces sufficient lift for flying. If there isn't enough runway there for a plane to accelerate, have an engine fail at the critical moment, and either stop or take off as appropriate, then the pilot cannot attempt the takeoff.

The easiest factor for the pilot to manipulate in this situation is takoeff weight. If the plane is lighter, less lift is required. This may give the pilot the option of taking off with fewer passengers, less baggage/cargo, or less fuel. This explains all of the behavior you are likley to see. Sometimes (particularly on small t-props) the airline will not load all the baggage. Other times, they will seek passengers to take a later flight. Yet other times, they will take off with less fuel ... leading to the extra fuel stop on some trips.

The choices for any given aircraft are laid out in a series of performance tables for each aircraft. There is not, therefore, a single magic temperature, although I can certainly imagine that when the temperature gets over 120 degrees in Phoenix, there aren't very many planes that could possibly take off.

This also explains why you don't see very many t-props flying out of Denver. Most tprops don't have enough power to carry sufficient revenue generating loads out of the mile high city in the summer.

Tom

Well put tgrove. Sounds like you are a pilot. Being a private pilot I have personal experience in this matter. I won't get into the story but let me say that a non turbo Piper Arrow taking off at a 7600 ft altitude airport is a real "experience". Density altitude is real.

tgrove is on the money. in some middle eastern countries, not to mention Nellis AFB, among others, the runways are more than 20,000 ft. because it can be 120 degrees F or more on the field. But there is no magic number, it depends on a lot of things, but having a long runway give you a little more room to finagle the other factors.

[This message has been edited by jet (edited 08-10-1999).]

One clarification regards Nellis AFB: both parallel runways (03L/21R and 03R/21L) are only about a little over 10,000 feet in length. Edwards AFB in California has one concrete runway that is nearly 15,000 feet in length, but has other runways on the dry lake bed that extend over several miles.

If I remember correctly, when Mexicana first used 727s out of Mexico City, they were equipped with JATO (jet(actually rocket)assisted takeoff) bottles to improve the hot and high performance, especially with the high altitude of the airport.

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AlphaSigOU
Causa latet vis est notissima - the cause is hidden, the results are well-known.

This is a timely post! Just yesterday I sat on a TWA (TWE) Jetstream Turbo in Columbia, MO, waiting for take-off to STL for my connection to SEA. We sat and sat in a very warm plane on the runway for quite sometime. Suddenly, the door reopens and a woman ducks into the plane and speaks to a man seated in the exit row. They apologize saying they were sorry but they did everything they could but he would still have to get off the plane.

Everyone on the plane is of course staring at this turn of events and the man leaving the plane jokes about it saying, "Are they taking me back to jail?" The door closes after him and the plane is now abuzz with interest. Finally the pilot comes on and explains that they had to reduce the weight because of the heat. What is interesting is this man weighed less than 200 lbs. so their calculations must be very close! We took off successfully, so that I could then sit on a TWA 757 at the gate in STL for an hour while they "worked on a logbook."

Unless I miss my guess, the FAA has decided that one person weighs 170 lbs. There's also a standard allowance for carry-ons. I don't know if checked baggage is weighed, or is there is a standard for that as well.

You are right that the numbers are very tightly monitored. I remember one flight in which the change in wind direction from flight planning to take off time made a flight no longer possible. I also remember one flight in which we sat in the run-up block for a few minutes, burning of the last few hundred pounds of fuel to make the flight legal.

Tom

AlphaSigOU, you are right, Nellis's two runways are 10119x200 and 10051x150. Hmmm, I must have been thinking about some other test site or something...

Baze,

I am not a pilot, though that's something I plan to do in the future, and I certainly have more than a passing interest in the topic.

Tom