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Plane Answers: Zeroing in on a few airplane systems
On a recent round trip on board 757-200s, when we would descend there was a sound like air leaking out of a tire coming from the engine area. It would last from 1-3 minutes in some cases or a matter of seconds at other times. What causes this noise?
Air conditioning and pressurization on an airliner both originate from what's called bleed-air that comes from the engines. On the 757, as the thrust is reduced to idle for descent, another valve opens allowing 'high-stage' bleed air to supplement the existing air.
This hot air is then run through air conditioning "packs" that heat or cool the cabin and provide pressurization.
You may have been hearing the rushing air associated with this valve.
Not being a mechanic, that's the best explanation I can provide. Any mechanics out there want to tackle this one? Big Ed?
While working, I flew Northwest from MSP to AMS monthly. When flying DC-10s, the cabin display showed the same flight level the captain reported - 35,000 feet = 35,000 feet. Now flying the A330, the captain reports 35,000 feet but the IFE reports something like 34,824 (a guess, not the actual number). Does the IFE not get its travel info from the plane's instruments? Since FL350 is about 10,600 meters, does the IFE work in metric units (rounded to the nearest 100 meters) and convert to an exact English equivalent i.e. 34,777 feet?
On older aircraft such as the DC-10, I can't recall where the inflight entertainment screen pulls the altitude information from, but the newer jets all use GPS data to display the altitude on the map back in the cabin. In addition to locating an airplane's position on a map, the GPS can also compute an approximate altitude.
Since we refer to an altimeter in the cockpit that's entirely based on air pressure, our version of 35,000 feet actually varies slightly depending on whether we're flying through a low or high pressure area.
It's not important that we're exactly 35,000 feet above the ground, but it IS important that we're using the same measurement as all the other aircraft. And this barometric altimeter is considered the most accurate way to establish our altitude relative to all other airplanes.
Down low (below 18,000 feet), we manually correct the altimeters for the variations in pressure with a small knob using a setting provided by air traffic control. This keeps us at a safe and known altitude above the terrain.
So you may notice that the altitude displayed in the cabin while in level flight below 18,000 feet is much closer to a given 1,000 foot measurement, such as 11,000 or 5,000 feet when temporarily leveling off during a climb or descent.
Michael goes on to ask:
Also, the system in the cockpit that warns of impending collision - does it work by getting transponder information from nearby aircraft or is the information derived from the ground? Over the mid-Atlantic, out of ground based radar coverage, is separation dependent on seperation leaving Canada and maintaining a constant Mach number or will the system warn if you are getting too close?
The TCAS system (Traffic Collision Avoidance System) works over the Atlantic, since it uses the transponder of each airplane to decide who, in the case of a controller or pilot mistake, needs to climb and who needs to descend to avoid a collision.
But you'll almost never hear that warning over the Atlantic since the Gander (Newfoundland) and Shanwick (Shannon, Ireland and Prestwick, Scotland) controllers place aircraft on 'tracks' at a proper spacing and altitude that should stay relatively constant with an assigned speed for the crossing.
It's far more common to receive a TCAS alert while flying domestically or while in Central or South America. And since the introduction of GPS, which has effectively reduced our airways from up to 8 miles wide to a much more narrow .1 of a mile wide due to vastly improved accuracy, TCAS has saved numerous lives already, usually without passengers even knowing it.
Do you have a question about something related to the pointy end of an airplane? Ask Kent and maybe he'll use it for the next Plane Answers. Check out his other blog, Cockpit Chronicles and travel along with him at work.
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