Originally Posted By: Win
Astro:
How much fuel would typically be loaded on a commercial airliner? Only Enough to reach its first destination plus reserves, or did this flight include additional destinations beyond Beijing, and could have had fuel on board for those additional legs?
What is the fuel burn rate at cruising speed and altitude?
When overwater, how often are position reports made on HF radio (if they still are), and is there any way to cross check those reports or is it just up to the crew to accurately report a position?
I assume they do the compass / timepiece / speed thing to dead reckon and cross check position and not just rely totally on the other nav systems?
Win - For international flights, there are several methods of calculating the fuel required. They typically load burn + fuel burn to alternate (if required) + reserves + taxi fuel. Reserves vary, depending on the destination. For example, in LA, it's 10% of burn + 30 minutes endurance + Alternate fuel. In Auckland, or Hawaii, it's burn + 2 hours...but overall, you typically land on an international flight with about 90 - 120 minutes of fuel remaining in the tanks.
Pilots can't just "fill 'er up". Airplanes have a maximum takeoff weight (in the 747-400, it's 875,000#.) and a maximum landing weight (in the 747-400, it's 635,000#.) So, for our 747, you have to burn at least 240,000# of fuel to get down to landing weight...clearly, then, over any flight segment that's much less than the airplane's max range, it won't be completely full of fuel for takeoff... There's also this consideration: climb performance (on takeoff, over terrain, with an engine failure) is reduced if the airplane is heavy, so a lighter airplane is often safer, even if it has less airborne endurance.
I'm not a 777 guy, though many of its characteristics are similar to the -400, and 757/767 on which I am type rated. I've got access to 777 information, though...
A 777's cruise altitude for best range performance varies from 30,000 at max weight, to about 43,000 at low weight...Fuel burn at cruise altitude is far less than at low altitude, and again, taking a moderately heavy weight airplane at normal cruise, a 777 burns about 8,000#/hr per engine, but that number drops to about 5,000#/hr per engine at low weights and optimum altitude.
In terms of nautical miles per pound of fuel, being at 5,000 feet gets you less than half the range of being at cruise. (this is a bit of an oversimplification, it depends on the airplane's weight, the temperature, and the airspeed flown, but it's close enough for our purposes here)...so, a 777 loaded with fuel for 4,000 miles likely can't go farther than 2,000 miles if flown at low altitude. Less if it was being flown at a high airspeed (which burns more fuel per mile).
There are so many variables, that I can only speak in generalities...but jet airplanes guzzle fuel when flown at low altitude. In fact, at idle, on the ground, they're often burning about half the fuel per minute that they burn when going 540 MPH in cruise...
Position reports over water (outside of radar range) are done either via HF or via Satcom datalink known as CPDLC (for pilot initiated communication) or ADS-B (in which the aircraft just keeps broadcasting its position). Which method is used depends on what the controlling agency has for equipment. The 777 has all the capabilities. In fact, it has dual SATCOMs, 2 HF radios, and 3 VHF radios. The position reports are given at reporting points (compulsory) designated on the navigation charts. In some parts of the world, it's every 45 minutes to an hour...in some parts, it's much more frequently. Again, depends on the controlling agency.
There really is no way for the controlling agency to independently verify the airplane's position once you're over water (way over water, outside 100 NM). Whether CPDLC, ADS-B, or HF position reports are received, the controlling agency has only the report for information. The airplane is outside radar range.
Position in the 777 is managed by dual Flight Management Computers. They take inputs from 2 GPS receivers, air data computers, two VOR receivers (using both DME and azimuth) and 3 ring-laser gyros (called ADIRUs) and then calculate the airplane's position. When close to an airport, ILS position is added to the mix, but you get the idea: lots of inputs and, in fact, very precise results. The FMCs themselves perform DR navigation, using those air data inputs as a cross-check to the GPS and ADIRU-inertial derived data.
Pilots will cross check instruments, inertial (ADIRU) and FMC positions at each reporting point...but in all honesty, on the 777 (like the 747-400), the airplane is within 0.1 NM of where it's supposed to be all the time. The only time that it gets off is when the FMCs have been programmed with an error in the route of flight, and that's a big reason that pilots keep checking the airplane, to make certain that the route is still being flown as filed/cleared.
The days of compass/DR or even using a sextant are long gone. Airplanes that fly internationally have to have this precision in navigation to allow for closer spacing on crowded overwater routes. The North Atlantic, for example, has many, many routes that are close together. If airplanes weren't able to ensure that degree of navigation (and altimetry) accuracy, then they would not be allowed on those routes because they would risk blundering into each other. Some airplanes don't have the modern navigation equipment, so they're stuck at lower altitudes or offset hundreds of miles to keep them out of the flow of dense oceanic traffic patterns that rely on precision for safe separation.