# Aviation questions

At what point does an airplane take off from the ground? What happens at this particular moment that makes the plane ascend?
Why do we have to uncover the windows when taking off and landing? And why not use parachutes to save the live of passengers when accidents happen?

To answer your last question first, where would you store enough parachutes for everyone? Other than in the movies, it’s a lot harder to use a parachute safely that you think. Three hundred people jumping out of where? Without getting the lines tangled? Just impractical.

Back to the first and second questions: When lift exceeds drag. At sufficient air speed and at the appropriate angle, the shape of the wing causes higher air pressure below than above. Did you ever stick your hand out the window of a car going 60 MPH? Change the angle slightly and you feel the air push your hand up or down. Same principle.

Third question: Do we really? I don’t recall being told that. In fact, I know I’ve sat next to people who, to my dismay, keep the window shades down during takeoff & landing.

[QUOTE=EverLearner]
At what point does an airplane take off from the ground? What happens at this particular moment that makes the plane ascend?

[quote]

Air going over the wing generates lift. The fast the wing moves through the air, the more lift is generated. When the airplane is moving fast enough that enough lift is generated to pull its weight off the ground it takes off.

[quote\Why do we have to uncover the windows when taking off and landing? And why not use parachutes to save the live of passengers when accidents happen?[/QUOTE]

Gosh, you know, there have been threads about that already, but I’m going to defer posting them to someone else or to tomorrow when I’ve had some rest and my brain isn’t so foggy.

And I think that screwed up post coding demonstrates why I should be off to bed instead of on a message board right now…

1. Lift has to exceed gravity but the pilot generally has some control over the exact point of liftoff. The elevator resides at the rear of the horizontal stabilizer. The pilot uses it to control the pitch of the airplane. About the time lift exceeds gravity, the pilot pulls back on the stick or yoke, the nose rises, and the plane takes off. There is some degree of discretion however and the pilot may force the plane in the air when it is marginal or keep in on the ground building up more speed before taking off. There are guidelines for the right takeoff airspeeds for every plane.

2. Most people think that the FAA has a regulation that seat backs have to be up and the shades opened. Many of us knowledgeable about aviation have never found such a thing. It is an airline policy that is most likely intended to provide outside visibility and light in the event of an accident and sudden evacuation.

3. There are more problems with parachutes in airliners than you ever knew. Some small planes have whole plane parachutes as options however.
A) Most accidents occur suddenly during takeoff or landing.

B) Parachutes add weight, they would have to be tested, and there is no good place to put them.

C) You shouldn’t jump out of a 747 at 30,000 going 600 mph with little oxygen and -70F temperatures alone let alone with your 300 closest friends. You couldn’t get the door open anyway unless you depressurized the cabin and then you would die from that instead.

D) If the pilot can control the plane well enough to get down to a safe altitude and fly around while 300 people take their first skydive, the plane is still the safest place for you to be.

E) Inexperienced people shouldn’t be tempted to jump en-masse to their deaths because they don’t understand parachutes.

F) DB Cooper type stuff.

Agreed. I’ve said this before: If it’s a federal regulation, show me chapter and verse in the FARs. If anyone does, then I shall stand corrected.

It may be a nitpick, but takeoff happens when lift overcomes weight - not drag or gravity.

The window shades open are, as someone already said, because of light levels. If it’s bright sunshine outside and you have to evacuate they don’t want you standing at the top of the slide squinting and blinking to get your eyes accustomed to the light. It’s the same reason they dim the lights on takeoff at night, so you can see better in the dark.

From my viewpoint in the event of a crash the burning jet fuel will provide more than enough light, but no one ever asks me.

As I recall, the policy was instituted following a 727 accident in Salt Lake City. The 727’s high T tail can result in difficulty in reducing the sink rate and this particular 727 landed short of the runway. During evacuation people had a hard time getting out of the emergency exits because trays and seat back blocked the exit. In addition, some passengers who were reclined were catapulted forward by being slid out of their loosely buckled seat belts. In addition people were injured by being thrown into their tray or the back of the reclined seat ahead of them. I believe a requirement to securely stow carry on stuff was instituted at the same time because of injuries resulting from loose junk hurtling through the cabin. All in all it was a mess and the decision was made to clean things up in preparation for landing, or takeoff, in the interest of safety in a survivable crash.

As others have said, the plane takes off when lift equals or exceeds the weight. With conventional landing gear the usual technique is to raise the tail to a slightly nose high attitude turing take run and when there is enough airspeed the plane will simply fly off the ground. With a tricycle gear, in the old days, the same method is used. On takeoff run slightly raise the nose and when ready the plane flies off the ground.

It gets more complicated with the big jets. The technique seems to be to hold the nose wheel(s) on the ground until the calculated flying speed, and a little more, is reached, then rotate the nose up and the plane takes off. The takeoff speed has to be checked each time because it depends upon the weight of the plane, the altitude of the runway and the runway temperature. I don’t know this for a fact, but I suspect the pilots have charts that allow them to find the correct speed and they check them carefully before takeoff. Especially on tight runways and at high altitude on hot days.

As to seat backs, try 14 CFR 121.571(a)(1)(v)(A) http://a257.g.akamaitech.net/7/257/2422/14mar20010800/edocket.access.gpo.gov/cfr_2003/14cfr121.571.htm

Admittedly it says the passengers must be briefed to have seatback upright, not that they have to actually do it. But there are other places in Part 121 that say passengers must comply with all crewmember instructions.

Further 14 CFR 121.577(b) provdes that tray tables must be stowed, while 121.577(e) provides that passengers must comply. http://a257.g.akamaitech.net/7/257/2422/14mar20010800/edocket.access.gpo.gov/cfr_2003/14cfr121.577.htm
I do agree that the window shades up requirement is not regulated at all; the airlines invented that one themselves, perhaps acting on an NTSB post-accident recommendation the FAA has studiously ignored for years.

And it’s a damn good idea to have shades so you can see out & decide whether you really want to open that door on the side with all the fire out there (thereby letting it all in here) versus opening doors on the other side which may well be clear.

If it is not too far OT, do they actually know the total weight or do they estimate?
Is there some sort of a scale in the landing gear?

The old “Learstar” ( A converted Hudson bomber with R-2800 attached) had a neat gizmo. ( Fast birds for their time at about 350 MPH )

It looked like a briefcase and inside were dials for all the seats, fuel tanks, oil tanks and luggage positions. There two large gauges with needles, one for total weight and on for center of gravity.

With all the dial turned back, the weight needle was on empty weight and the CG needle was on the empty CG. As you added fuel, oil, people in each seat, the weight needle would creep up and the CG needle would go back and forth depending on what was being added where. It was too cool and worked great.

It was all done with strings and pulleys and was most ingenious. To day we have hand held computer programs to do the same thing.

I do not have a ‘cite’ but I have heard of some planes having ‘strain gauge’ type scales tied into computers set up on the landing gear just for the purposes of weight and balance. Seems a no brainier to me but it does not seem to be universally adopted.

Regarding takeoff speeds and distances, there are several critical speeds associated with takeoff and initial climb. Because they depend on the weight of the airplane, and that can vary by hundreds of thousands of pounds, they are calculated each and every time a large aircraft takes off. In small airplanes, the difference in weight from empty to max gross is just not that much. Our speeds would only change by a few knots, so we just use the speeds that are correct for our maximum gross weight. Operation at any weight at or below max gross will be safe at those speeds. However, every airplane in the world has performance charts used to calculate takeoff and landing distances, including the small ones I instruct in. Changes in atmospheric pressure, temperature, airport elevation, runway gradient, surface type, weight, and wind can make a big difference in required takeoff and landing distances.

As for calculating the weight of an airliner, the dispatcher knows how much the “empty” airplane weighs (airframe, fluids, unusable fuel, etc). He/she also knows the weight of all the cargo and baggage being loaded, and the FAA has standard passenger weights to use for people. I believe it is 185 lbs for males in the winter months and ten pounds less in the summer months. I can’t remember the standard female weight. This stuff is all in parts 121, 125, 129, and 135 of Section 14 of the Code of Federal Regulations.

As other posters have said, the speed at which an airplane lifts off is when net vertical lift exceeds weight. Without getting too technical, lift depends on two things. These are speed and the angle of the wing relative to the air moving past the airplane. Picture the angle of your hand relative to the wind rushing past your window. This is called angle of attack. The slower I go, the more angle of attack I need to create sufficient lift to keep my hand (or airplane) flying. However, as I slow down, I will eventually get to such a high angle of attack that my hand will fall again, and my arm hits the window sill. Replace my arm with an airplane and the window sill with the runway. You get the idea. So sure, I could rotate agressively at 40 knots and the airplane will climb on the verge of a stall, or I could wait until I build up a bit more speed so I have a little cushion to work with in case my headwind dies just as I rotate.

Irrelevant. An air carrier practice or procedure does not have to be specifically laid out in 14CFR for it to be an FAA requirement.

Thank you for reminding us of this. To expand, each air carrier comes up with its own operating specifications, and submits them to the appropriate regulatory agency (the FAA in the US). Once approved, these op specs are essentially the regulations under which that carrier operates.

Let me be the first person in this thread to mention a treadmi…

Nononono, I can’t do it. Just can’t do it.

This is probably the definitive thread on why airliners don’t have parachutes

Except that I’ve heard people claim that opening the window shades is an FAA regulation. If it is, then they need to provide a cite. I’m not questioning whether it is or isn’t a good rule; only whether it is a regulation as some people claim.

They know the weight of the aircraft, the number of seats, and they know the weight of the fuel. They also know the mandated weight of the bags (the weight limit is 50 lbs.) and the average weight per person (I will venture no guess so as to avoid controversy on this point).

If you have 300 people, each with two bags, you have an estimated max weight of 15,000 pounds for the baggage. Add the weight of the passengers to that, the plane, the fuel, and the baggage and you have an absolute worst-case weight equation.

Why worry about getting it exactly right when you can build a safety measure into the equation by going with the worst case scenario? That even accounts for last-minute passengers, because surely a few people cannot exceed that estimate.

That’s how loadmasters do it in the military, anyway. I can’t imagine that commercial operators would try to be uber precise given the penalty for being wrong.