Plane Answers: Takeoff speeds, weights and lavatory drains

We’re combining five questions that were recently submitted for Plane Answers. We’ll look into who foots the hotel bill for a crew’s layover, what is the typical speed and weight at takeoff, how pilots line up with a runway visually and where does that lavatory sink water go inflight?

When pilots layover for a night before returning home, who picks up the bill?

The airline picks up the tab for each crewmember’s hotel room. Meals are up to the employee, although often there’s a small per diem of about $2 an hour that’s paid by the company to cover these expenses.

I’ve always wondered what’s the ideal speed for a plane like a 737 to takeoff.

The takeoff speed is based on the weight of the airplane which varies. But you can think of it as a speed between 135 and 155 knots or so. Add 15% to convert knots to m.p.h. and you’ll have a liftoff speed of around 155 to 178 m.p.h.

What is the maximum weight that the average passenger plane (737,757 etc.) can carry and be able to takeoff and remain airborne? As a frequent flyer, I become concerned when I observe a number of 300 pound passengers boarding; and then there’s the cargo below. On occasion, I have been on a plane where the weight load was so light, passengers were asked to shift around to balance the plane weight. Does the opposite ever occur?All U.S. airlines use an average passenger weight of 190 to 195 pounds depending on the time of year. In the FAA’s mind, people in the winter either put on weight after the holidays, or they just wear more clothing. The load planning computer is able to keep the weight distribution spread out evenly through the cabin for balance purposes. The cargo is weighed before it’s loaded on to the aircraft so we know that number to the exact pound.

All of this data is totaled and sent back to the pilots who make sure that the actual weight is below the maximum allowed either structurally by the airplane’s manufacturer, or below the maximum that the runway will allow based on the airplane’s performance and the outside temperature. As an example, a 757’s maximum takeoff weight, assuming it’s not limited by a shorter runway, is 250,000 pounds. It’s empty weight, without fuel or passengers, is around 130,000 pounds.

When flying the final approach visually, how do you line up the runway centerline visually?

It’s not unlike lining up a car when turning onto a road. Controllers usually give us an intercept angle of no more than 45 degrees or so, and it’s our job to join the imaginary extended centerline from the runway. Once on final, it’s not hard to see if you’re right or left of course. Small heading corrections using the ailerons to bank left or right and we’re perfectly lined up. We also use any available navigation aids (GPS, ILS, or Localizer) to cross check our position.

This is kind of a goofy question but I always wonder about it when I use the restroom on a plane. Why is the drain stopper always closed? I notice that when I drain the water in the sink there’s an “air” sound. Does it have to do with the air pressure in the plane?

The sink drain actually ‘vents’ out of a heated drain mast (to prevent it from freezing). If they left the plunger open, it would create a lot of noise, and use a tiny amount extra bleed air to pressurize the airplane, which I imagine would cause a small decrease in fuel economy.

I took this video that shows a 777 inflight when someone is draining the sink. Take a look–It’s at the 1:14 point:

Have you ever been curious about what goes on at the pointy end of an airplane? Ask Kent and maybe he’ll use your question for next Friday’s Plane Answers feature.