By Barry Schiff
Turbofan-powered airplanes have two maximum airspeed limitations. One is VNE, the never-exceed airspeed limit familiar to pilots of all airplanes. It applies at low altitudes where the atmosphere is relatively dense.
Another limit is MMO, the maximum operating Mach number, a “redline” for high-flying jets.
If one flies too close to the speed of sound, shock waves and compression can cause serious consequences (depending on the airplane) such as unusual handling qualities, irregular aircraft movement, aileron buzzing, aileron snatching (the ailerons move abruptly and uncontrollably in one direction and the other), large and violent trim changes, deterioration of control effectiveness, and buffeting so intense as to cause structural failure.
Pilots operating jet aircraft, therefore, must abide by limits based on their proximity to the speed of sound.
An interesting reason for these adverse characteristics is a surprising change in the behavior of air as it approaches the speed of sound. Unlike subsonic air, which accelerates and is subjected to reduced pressure as it flows through a venturi, supersonic air does just the opposite; it slows down and is subjected to increased pressure.
By: Vincent Czaplyski
When 24-year-old Capt. Charles “Chuck” Yeager first officially punched through the sound barrier on October 14, 1949, the event was marked by the jump of his aircraft’s Machmeter needle from Mach 0.965 to Mach 1.06. The brief flirtation with supersonic flight (the rocket-powered Bell X–1 Glamorous Glennis exceeded the speed of sound for just 18 seconds) assured Yeager’s place in the annals of aviation history and paved the way for further rapid advances in high-speed flight.
The sound barrier has long since lost its ferocious reputation as a man-eater, but the need to accurately determine a jet’s speed at high altitude remains as important today as it ever was. For pilots, the means of doing so is the Machmeter.
The Mach in Machmeter is named for Austrian physicist Ernst Mach, who, in an 1877 paper dealing with supersonic velocity, described a method of measuring an object’s velocity relative to the speed of sound. The speed of sound through the atmosphere is not a constant but varies by temperature and density (that is, altitude). For example, at sea level on a standard day it is about 761 mph, while at FL370 it is 660 mph.
A Machmeter accounts for these variables and displays an aircraft’s velocity as a percentage of the speed of sound. While some older Machmeters only displayed Mach, modern instruments usually combine indicated airspeed and Mach indications in a single unit.
Most civil jet aircraft cruise within a Mach range of approximately Mach 0.72 to Mach 0.86, with some notable exceptions such as the Cessna Citation X, which can cruise at Mach 0.92, and the Eclipse 500 at Mach 0.64. All are limited, however, to a maximum operating Mach number—MMO—which when observed ensures that localized airflow over the aircraft’s wings will not reach supersonic speeds even while the aircraft itself is subsonic. This occurs at an aircraft’s so-called critical Mach speed, and exceeding this speed can create undesirable aircraft control issues.
MMO is a constant number for a particular aircraft type. (For example MMO for the Lear 35 is Mach 0.83.) But its depicted position on the face of a Machmeter relative to a given airspeed changes as the speed of sound changes with altitude. It is thus depicted on a Machmeter by a moving needle commonly known as the “barber pole,” so named because it is usually painted in an attention-getting, red-and-white-striped barber-pole pattern.
During climbs and descents in jets pilots reference either airspeed or Mach, and the transition between the two usually occurs somewhere around FL270. Controllers will assign a “speed” below this approximate transition point and will generally specify a Mach number above it, as in “Lear Three-Seven-Delta, when transitioning from indicated airspeed maintain Mach 0.75 in the climb.”
As a practical matter the transition point is really based on one’s planned indicated airspeed and Mach in a climb or descent. Where the two are equivalent for a particular altitude (for example, 300 knots equals Mach 0.76) the transition has been reached. From that point the pilot would maintain 300 knots (in a descent) or Mach 0.76 (in a climb) and the other value would continuously change.
Below the Mach transition range, a climbing aircraft’s true airspeed and Mach number will increase for a given indicated airspeed. On a recent flight in a Boeing 767-400 our planned climb speed was 340 KIAS leaving FL210, equating to Mach 0.742 and a TAS of 456 knots. Passing FL250, Mach had increased to 0.801 and TAS to 481 at the same 340 KIAS.
By FL260 we had transitioned into the Mach crossover regime, as witnessed by the indicated airspeed dropping to 330 KIAS while Mach number and TAS hovered right at 0.801 and 481. From that point all the way to FL330 our Mach number remained constant while both IAS and TAS continued to drop.
Vincent Czaplyski holds ATP and CFI certificates. He flies as a Boeing 757/767 captain for a major U.S. airline.
By: Thomas A. Horne
Let’s say you’re flying along and then your day goes very, very bad—very quickly. The annunciator panel lights up with discouraging news: you’ve lost both your engine-driven generators, and have only the battery to provide electrical power. That means you have about 30 to 45 minutes to make it to an airport before the cockpit goes dark. That’s cutting it very close. This news is even worse if you’re over desolate terrain, or over water. The nearest airport may be far, far away.
If the airplane has an auxiliary power unit (APU), its generator can be used for electrical and hydraulic power. But what if you’re flying above the APU’s operating envelope? Usually, this envelope tops out around 25,000 feet. You must fly below this altitude in order to start and use the APU. But down this low, fuel consumption rates rise dramatically, compounding the already tense situation. And what if the APU isn’t running the moment both those generators quit?
Some jet manufacturers preempt this dilemma by designing the airplane with a ram air turbine—sometimes called a RAT or an ADG (air-driven generator). The RAT is a small, propeller-powered generator that can put out enough electricity to work essential cockpit flight instruments and radios, plus landing gear, flaps, and other components critical for making a safe landing—even in instrument meteorological conditions.
Here’s how the RAT literally springs into action. First, the moment that generator power is lost, battery power is sent to the RAT deployment control unit via the battery bus. Within four seconds or so, the RAT’s storage compartment door opens and the unit drops down into the free air stream. Then the RAT’s propeller immediately begins to spin up. Rotation speeds of 12,000 rpm are not unusual, and there’s a propeller governor to keep rpm steady at the desired level.
There’s a deployment speed envelope to keep in mind—for extra insurance against propeller under- or overspeeds. Typically, the RAT will work at any altitude, unlike an APU. (Some APUs are only approved for use on the ground.)
The ideal setup would be to have a RAT that automatically deploys in those rare instances where both generators conk out and the APU is off. Bombardier’s Challenger and Global series of business jets have this system design. It’s proven so reliable that these airplanes can be dispatched without an operating APU—because the RAT is always there, ready to answer that rare, desperate call to action.
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