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gura75

Magical 10000ft?

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How come 10 000 ft seems so "magical". Fasten seatbelt sign comes off when passing 10 000 ft. You can switch on your electronic equipment like laptop, cd player etc. You can even startsurfing internet when crossing 10 000 ft. The same thing happens when descending and crossing 10 000 ft. You fasten your seatbelt, you turn off your electrical equipment. How come it is 10 000 ft.I presume the T/D starts much earlier. How come we do not fasten our seatbelt at T/D? gura75

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I think it's the same reason as why you're not allowed to exceed 250 knots below 10000ft: you're in busy airspace where you have to be actively separated from other aircraft, so the airplane may make more turns, descend and/or climb at a higher rate, et cetera. Above 10000ft you're in quieter airspace, so flight is expected to be smooth and you can unfasten your seatbelt.Also, below 10000ft your landinglights should be turned on. Again, this is because you're in busier airspace where extra visibility is desirable.

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There are historical as well as practical reasons for 10,000 feet being a magic number. That's the altitude where a typical human will need to go on oxygen to remain completely comfortable, although as far as human medicine goes, 5,000 feet above sea level is regarded as where high altitude starts. Most cabins are pressurised to replicate 8,000 feet when an aircraft is in the cruise as this is a good balance for structural integrity without having to over-engineer aircraft skins and where humans will be comfortable if not moving around too much.As far as hypoxia goes (i.e. medical problems due to a lack of oxygen), a really critical altitude is in fact 23,000 feet, since that is about the limit where you still have a reasonable amount of time to get an oxygen mask on and turn on the flow before you'd risk hypoxia making you not conscious enough to care about doing so before you passed out (i.e. that time is about 40 seconds). It is for this reason that the original rule about flight crew donning oxygen masks when above 25,000 feet if one of them left the flight deck was established. Interestingly, what most people don't know about that, is that it is not the thinness of the air that is the main issue, but rather the pressure; when breathing, you actually breathe in by relaxing your muscles, whereupon the air pressure itself is what pushes the air into your lungs rather than you drawing it in, and above 23,000 that does not happen because the pressure differential between the atmosphere and your lungs is not enough to allow it to occur. If you read any WW1 fighter or bomber pilot autobiographies, you will see that patrols and missions were in fact regularly carried out at 18,000 feet without oxygen, although it would often leave the crews very tired after having done so, and some WW1 aircraft did actually have oxygen systems because of that (principally German reconnaissance two seaters such as DFWs).Up until the 1950s, pretty much every airliner was unpressurised - there were some exceptions, such as the Lockheed Constellation of 1940s vintage, but it was certainly unusual in that and WW2 put a stop to it being fully developed as an airliner until hostilities ceased. This meant that 10,000 feet (as a nice round number that is easy to recall) became fixed in the minds of most aviators as the limit for oxygen by the time airliners were commonplace, and it is a good sensible margin in medical terms. But there are other reasons for it too. Unless there is bad weather about, 10,000 feet will put you above most fair weather cumulus clouds and give you enough slant range visibility to pick out landmarks for VFR, this notion being based on an era where even airliners used to follow roads, rivers and railway lines, which is why you are supposed to fly on the right of these features and also why many structures in the US have the name of the town painted on their tops.As far as using laptops and such is concerned, again it is a convenient number for airline operations, whereby the crew have an easily remembered transition point for things to happen, including more critical things such as turning on the Landing Lights if required to aid visibility.As an airliner approaches the T/D, the crew usually have a pretty good notion of what the weather will be up ahead, since they can be pointing the radar at it from several hundred miles away, so they will generally have a good idea whether it is going to get bumpy or not, although there is of course no guarantee that this will be the case. It is at the discretion of the crew to say when you should put your seatbelts on, but you will note that they generally ask people to do so at all times when seated, which is in fact a sensible precaution since it is a fact that several people are killed on airliners every year from injuries incurred in turbulence when they were not wearing a seatbelt. On a climbout after take off, it is more difficult to know what the weather up ahead will be, since the aircraft may have to fly a complex departure pattern which will make pointing the radar at where they are going more difficult. Airliner radars do compensate for deck angle tilt, but you still have to be pointed more or less where you want to check out the weather for them to be of any use, so they'll simply err on the side of caution and tell everyone to stick their seatbelts on when departing, as it is one less thing for the crew to worry about until they are established in the cruise or at least heading that way.Al

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I think generically, the navaids used in terminal procedures have a weaker signal than enroute navaids. As such the consumer electronics stuff may cause interference with some of the flight deck instruments used below 10,000 feet in these dense traffic areas and where navigation is more critical with some aids being weaker with less course tolerance and packed more densely. Later consumer products have better shielding but that is not all encompassing for the navigation and communication frequency spectrums used. On night flights passenger cabin lights are also turned on waking passengers (probably at a higher altitude in descent) so passengers are alert to respond to any crew instructions and ready for normal or emergency deboarding. They don't want distractions in the passenger cabin.As I recall from my commercial GA pilot training in the early 70's 12,000 feet density altitude was the physical limit for oxygen alertness for the pilots being a major factor. 12,000 MSL was the limit for non-pressurized aircraft without oxygen supplementation by VFR rules.On a cross country trip (Minneapolis to Bridgeport, Connecticut VFR in three major legs) I did get up to 10,000 MSL and did not get goofier than normal :)

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