I did after once flying out of Denver, and the luggage had to be removed because the airplane was overweight for the altitude and air temperature.

Next time you're flying out of Denver, notice how long the takeoff roll is. When my father flew jets out of Denver, he'd hang the tailpipe over the back edge of the runway so he'd have every last foot of runway to get airborne.

The difference in Denver is so significant that United actually swapped all the Houston-based 737s that came over from Continental in the merger with the Denver-based A320s from United. The 737 base moved to Denver and the A320 base moved to Houston.

The 737 has better performance with high-altitude takeoffs.

I never get tired of watching this video of a "vodka burner" using every inch of runway for takeoff. https://www.youtube.com/watch?v=PLxEHIbHUlY

How does the pilot know the weight? Do they weight each piece of luggage individually, or are there sensors in the wheels?

Good question! Here's some information from aviation.stackexchange: http://aviation.stackexchange.com/questions/9975/do-addition...

Checked luggage is weighed when you drop it off, so presumably the exact weights are available to the pilots. For passengers themselves and their carry-on baggage, accurate weights aren't available so statistical averages are used, based on the number of passengers and their baggage allowances.

Airliners do have "squat sensors" in the landing gear, but they're not capable of accurately measuring the aircraft's weight. They're just binary-valued sensors that check whether the gear struts are under stress, to prevent the gear from accidentally being raised while the plane is on the ground.

I wonder if gluing strain gauges to the landing gear struts would be accurate enough once calibrated.

There has been at least one crash due to mis-estimating the weight of the passengers + luggage + cargo. A strain gauge on the struts, even if inaccurate, could provide a backup sanity check for the weights.

As I joined FedEx, I was told that they had already worked out sensors, of some kind (I never saw details), on the landing gear that would give the data needed for knowing and controlling weight and balance. For balance, right, e.g., don't want all the weight near the tail and, instead, want the weight more evenly distributed in the cargo area along the length of the plane, that is, want the load in balance.

So, yes, FedEx did think of having sensors, of some kind, in the landing gear.

But I never heard more about such sensors, and, for the times I rode the jump seat in the planes, I never saw pilots working with weight and balance from data from the landing gear.

Maybe later, after I went to grad school, FedEx did do something with such sensors.

So, right, the sensors are a good idea and were considered and, maybe, eventually implemented.

They could put something in the ground - a weigh station - that the plane travels over before getting to the runway. That avoids the need for modifying airside equipment which is a bureaucratic nightmare.

I don't know what happens in the USA, but in Australia each piece of luggage is weighed before it can be checked in. For various reasons: heavy bags (over ~20kg) must have warning labels attached, very heavy bags (over 32kg) are disallowed with no exceptions, and it means a total can be provided to the pilot.

Significant. I'd have to go upstairs, find the right box, get out the flight manual (Fred had a guy send me while I was doing my Ph.D.), and learn how to use it again to answer.

But, yes, hot air at high altitude hurts jet engine thrust, a LOT.

I believe that pilots refer to this as density altitude. It factors greatly into allowable takeoff weight, takeoff distance and ability to climb over obstacles in the takeoff path. Pilots are or should be very aware of the density altitude on very hot days especially at high altitude.

An example of this I was recently explained:

There's a leg connecting Tokyo and Mexico City. The Tokyo flight goes direct to Mexico City however the Mexico City flight stops in Hermasillo on the way to Tokyo.

Due to the higher altitude of Mexico City, it takes more fuel to get off the ground and requires a pit stop in hermasillo to get all the way to Tokyo.

Disclaimer I'm only partially sure that I'm remembering the whole story correctly.

You mean Hermosillo.

Heat is a factor too.

Every couple of years, there'll be a day or two in summer when temperatures at the airport in Phoenix get above the maximum takeoff temperature for a lot of the smaller regional jets, and chaos ensues as all those flights (it's a US Airways hub) delay or cancel.