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The Mighty Boeing 747-Jumbos – Part 5

Excluding the Concorde, which was conceptualised, designed and built as a supersonic aircraft, the Boeing 747 Jumbos are the fastest airliners in the world. They are capable of easily maintaining a cruise speed of Mach 0.86 for hours on end, albeit that this speed is slightly faster than the speed that gives the best economy. Depending on the weight of the aircraft, their usual cruise speeds average out at about Mach 0.85.

If requested by ATC to fly at a faster speed in order to effect a greater separation from other traffic, the speed can be increased, both safely and legally, to as much as Mach 0.90. There is no other subsonic/transonic airliner in existence that is able to match the 747 for speed, although the Boeing 727, now almost totally extinct, comes a fairly close second.

The actual Maximum Mach Operating Speed (MMO) for the Boeing 747 series of aircraft, is  Mach 0.92 ! This is a stunning 92% of the speed of sound, only Mach 0.08 away from the magical figure of MACH ONE!!

However, no airliners are ever operated at speeds that are near their MMOs. There are a number of covert and somewhat unpredictable dangers and traps that lurk there, such as Mach “buffet”, Mach “roll off” and then also, perhaps, some abnormal pitching moments. One or more of these phenomena could quite easily cause difficulty in controlling the aircraft. Instead, for the most effective cost-saving  performance  and adequate manoeuvring capability in rough air, airliners are flown at speeds that are about 6% to 7% below their MMOs.

For interest’s sake only, we find that when comparing the MMOs of  various different airliners, that the 747s are the fastest at Mach 0.92. In second place, at Mach 0.9, is the lovely to fly  Boeing 727 and third is the SUPER LARGE Airbus A 380 at Mach 0.89.

Three long-gone civilian airliners that no longer grace the skies are the Douglas DC8, the Convair 880 Coronado and the Boeing 707. The DC8 was once test flown to Mach 1.01 and had an MMO of close to Mach 0.92. The other two aircraft both had MMOs of Mach 0.884. After these airliners, in a slightly blurred descending order, are the Airbus A 300 and the A 340 series at Mach 0.86 and then the Boeing 737 series together with the Airbus A 320 series at Mach 0.82. 

 Mach Number is simply the ratio of the true airspeed of an aircraft, to the local speed of sound. Due to the fact that the molecular activity of air is higher in warm air than it is in colder air, the speed of sound will be higher in warm air than it would be in cold air.

The speed of sound at sea level on a “standard day”, when the ambient temperature is plus 15 degrees Celsius, would be 667 knots. However, in the “standard atmosphere”, at an altitude of 36 090 feet, the ambient air temperature would be minus 56.5 degrees Celsius, and the speed of sound would be a lot slower, at 574 knots. The difference in the local speed of sound (LSS) is 93 knots, which is a 14% difference. These values are based to a large extent on scientific measurements and assumptions, but it in actuality they are fairly close.

Playing around with the formula, Mach Number equals the true airspeed  (TAS) of the aircraft divided by the local speed of sound (LSS), we find that at Mach 0.86, in still air, the aircraft would be cruising along at a groundspeed of 494 knots and that at Mach 0.84, it would be doing  482 knots. These values give you “ball park” performance figures of 8 nautical miles per minute, which  is certainly something to write home about!

Thank you for reading this, my last article about flying the Boeing 747s and the 747-400 in particular. The whole range of them, with their various models, uses and power plants, were wonderful aircraft to fly. I have many abiding memories of doing  automatic landings in them, where the fog  so thick that there was no defined cloud base and where there was as little as 100 meters of visibility down the centre line of the runway. Then too, there were the operations in and out of snowy airports that had ice patches on the taxiways that could cause even a 747 to slide off into the “rough”.

If ever there was a phase of the operation that pushed a pilot’s pulse rate up it was a maximum all-up weight take-off out of a windswept New York on a dark winter’s night. The Standard Instrument Departure (SID) off the long Westerly Runway called for a turn immediately after take-off so that the La Guardia Airport’s airspace would not be infringed upon. It was a case of calling, almost within in a breath or two, “V1, Rotate, Positive Climb, Gear UP, Turn me left heading 170…all this before the aircraft had reached 200 feet above the airport level! The 747s took all of this in their stride! 

Above all, was the sheer delight of sitting in a spacious and comfortable cockpit, 12 kilometres above sea level and picking up a “same direction” jet stream that was blowing at speeds of 100 to 150 knots, thereby  pushing your groundspeed up to well over 1000 kilometres per hour and knowing that without any Concordes operating on the same routes, no other airliner stood a chance of catching up to what was and still is operated as the fastest subsonic airliner in the world!