The Flight: Climb, Cruise, and Descent: Airplane and Airline FAQ

STERILE COCKPIT RULE

When the plane is moving on the ground, and when it’s below 10,000 feet, the cockpit crew is prohibited by FAA law from any extraneous communication not relating to the safety of the flight. This is known as the Sterile Cockpit Rule. It’s intended to reduce pilot workload during the busiest portions of the flight. A literal interpretation makes announcements to the passengers—choppy air, unusual departure procedure, a plane nearby flying a parallel route, go-arounds on approach, and sightseeing narrations—all illegal when below 10,000 feet. There are many times a simple explanation would ease some concern. Pilots know this. But the Sterile Cockpit Rule prohibits it.

AIR TRAFFIC CONTROL (ATC) AND TRAFFIC SEPARATION

The only technique pilots have for avoiding a midair collision is to “see and avoid.” When the planes are close to airports, flying at relatively slow speeds and with good visibility, visual separation is safe. At higher flight levels and greater speeds, “see and avoid” does not provide anywhere near the margin of safety required. Consequently, the responsibility for traffic separation rests with Air Traffic Control.

As stated earlier, every plane is equipped with electronic devices that send ATC an exact location and altitude, accurate to within 100 feet. After a distinct four-digit code is entered into the on-board transponder, the flight is positively tagged and followed by the ATC ground computers. As the coded blip passes across the Air Traffic Controller’s radar scope, he can see both the plane’s exact current position and its anticipated location in any selected time frame. The current speed, direction, and rate of altitude change are all displayed and updated every few seconds.

There are 20 separate Air Traffic Control centers throughout the 48 states. Each center is subdivided into multiple sectors with different radio frequencies. Though pilots talk with one portion of an ATC center at a time, their radar blip can be seen on multiple radar scopes simultaneously. In fact, long before radio frequency is changed from one controller to the next, radar contact will already have been established.

CONFLICT ALERT (TRAFFIC SEPARATION)

In good weather all aircraft within the terminal radar area are kept a minimum of three miles apart. In bad weather, the distance is greater. Leaving the airport vicinity, the minimum separation distance increases to five miles.

With increasing speeds and higher altitudes, the minimum separation can increase to 10 to 20 miles. It is not uncommon during thunderstorm season, when a great deal of circumnavigation is required to stay clear of any intense rain showers, for the minimum distance to increase to 50 miles. The weather conditions and traffic congestion dictate the traffic separation necessary.

If the ATC computers ever sense that within the next two minutes airplanes will get closer than the prescribed mini mum limits, their computers will indicate a conflict alert. Both an aural and visual radar-scope warning alert the Air Traffic Controller to increase the separation. They would immediately communicate with one or both aircraft and direct them to turn, climb, or descend.

Air Traffic Controllers face discipline for allowing unexplained conflict alerts. Therefore, if the required minimum separation is five miles, ATC will keep them a significantly greater distance apart, allowing for an even bigger margin of error.

Traffic Alert and Collision Avoidance System (TCAS) is an additional safety feature that will be installed on all commercial aircraft by the end of 1993. At present, only ground-based radar sites ca receive and interpret transponder and encoding altimeter information. But the new TCAS will make it possible for all airliners to see the radar blip of all nearby planes long before they could be visually sighted. If a conflict is sensed by the airborne system, a warning will not only alert the pilots, but the two TCAS systems will interpret each other’s telemetry information and issue specific avoidance instructions.

PITCH ATTITUDE—CLIMB SPEED

As the airplane climbs through 10,000 feet, the most noticeable change in the cabin will be the pitch attitude. Up to this attitude, ATC limits all aircraft to a 250 knot (288 mph) speed restriction, since there are more planes in less airspace at the lower altitudes. When the 250-knot speed restriction is lifted, the pilots will accelerate to a more efficient cruise climb speed of 300 to 320 knots. To accelerate, both the climb power will be increased and the angle of climb decreased. The power change can be heard and the decrease of pitch felt as it lowers from more than 10 degrees nose up to a more level-feeling 5 degrees.

POSITIVE CONTROL AREA

Federal Aviation Regulations define two distinct methods of flying: (1) by visual reference only—looking outside—or (2) by instrument reference, where no outside visibility is required.

Regardless of visibility, commercial airlines are required to always operate IFR (Instrument Flight Rules). If visibility is good, it makes for a more pleasant flight, but being able to see outside is not necessary. Flying under IFR rules, whether or not visibility is good, requires full ATC clearance and positive transponder identification.

Above 18,000 feet the airspace is known as the Positive Control Area. All flights, private and commercial, must operate under full IFR rules. Most of the little private aircraft are not capable of flying that high, but many charter, corporation, and commuter planes are. To ensure the full traffic avoidance, ATC must be utilized. Above 18,000 feet no exceptions to the full IFR rules are made.

CABIN PRESSURE

The pressurization system is capable of maintaining sea-level cabin pressure up to a flight altitude of about 24,000 feet. However, if your cruising altitude is 35,000 feet, and the cabin pressure is held at sea level until 24,000 feet, all the internal cabin pressure change would be concentrated in the last 11,000 feet of climb, which might be uncomfortable. Instead, the cabin altitude begins climbing very slowly immediately after takeoff.

Rates of internal cabin pressure change are kept to 500 feet per minute while going up and 300 feet per minute while coming down. This is less than the rate of change in many high-rise building elevators. At cruising flight levels the cabin altitude averages 6000 to 7000 feet.

Some passengers find it harder to clear their ears on descent rather than when the craft is climbing. Flight attendants are trained in various helpful techniques, such as squeezing your nostrils and blowing lightly, swallowing, chewing, and placing warm compresses on your ears. These techniques work best if started early in descent.

STEP CLIMB

Airplanes are capable of climbing from takeoff to their initial cruising altitude without ever leveling off. But on most flights ATC restrictions prevent this. Pilots prefer a rapid climb to at least 10,000 feet because it gets you above most light aircraft and does not require large climb-power adjustments.

However, there are times, because of conflicting traffic and/or delayed radar hand-offs from one ATC facility to another, when altitudes are held down. These “hold down” altitudes result in step climbs.

Just like a set of stairs, planes climb and level off, climb and level off. If ATC clears a plane to 5000 feet, it will climb to that altitude and accelerate to 250 knots. Once level “at five,” a sizable power reduction will be made in order not to exceed the 250 knots-below- 10,000-feet speed limit. Once cleared higher, the climb power will be restored to maximize the rate of climb. If another hold-down altitude is issued, the power will be significantly reduced again and reapplied when further climb clearance is received.

As mentioned, all Air Traffic Control centers are divided into sectors, though some sector divisions are geographic and others are dictated by altitude. Climbing through 23,000 feet requires a transition from the low altitude to high altitude Air Traffic Controller. Since the high altitude controller has to fit you in with all the aircraft already at cruise, a temporary altitude limit of 23,000 feet is usually issued until higher altitude airspace is available. This requires another step climb.

As the plane climbs to the flight levels in the 35,000 foot range, step climbs are self-imposed. Heavily loaded air planes, with a full complement of fuel and cargo, may be altitude limited. When the weight of the plane is reduced by fuel usage, a step climb to a higher, more efficient altitude will be requested. Long overseas flights, with very heavy fuel loads, require multiple step climbs.

CRUISE ALTITUDE—PITCH OF THE PLANE

Approaching cruise altitude, the pitch of the airplane, which has been approximately 5 degrees nose up, will again be lowered to stop the climb. If cruising at maximum airspeed, the angle of pitch required to maintain level flight is small. Flying at speeds slightly slower than max cruise— for increased fuel economy—requires a slight nose-up pitch from 1 to 3 degrees. The pitch attitude is adjusted according to speed in order to maintain level flight.

Climbing through 18,000 feet, the plane enters the Positive Control Area, where all flights are conducted under IFR rules. Also above 18,000 feet, altitudes become Flight Levels (FL). For example, 25,000 feet becomes FL 250. The difference lies in how the barometric altimeter is set to measure altitude. Below 18,000 feet the altimeter must constantly be updated to reflect the local barometric pres sure, so a true measure of altitude above sea level is read on the gauge. Above 18,000 feet in the United States, terrain avoidance is not a hazard, except in some locations over Alaska. Traffic separation is the priority, so instead of having all the airplanes continually resetting their barometric altimeters, everyone uses the standard pressure setting of 29.92 inches of mercury. What this means is that FL 250 is not exactly 25,000 feet if the pressure is anything but standard. The amount of variation is exactly the same for all aircraft.

As planes fly through areas of changing pressures, their flight level remains exactly the same, but their true altitude above sea level gradually changes. They will be gradually climbing and descending, which is hardly detectable, how ever the power changes necessary to hold a constant airspeed may be noticeable.

NAVIGATION

Most aerial navigation is done on a point-to-point basis. A 2000-mile flight may involve ten 200-mile segments. In the United States there are over 700 ground-based VOR (Very High Frequency Omnidirectional Range) stations, all spaced at intervals. Connecting all these VOR stations are airways, highways in the sky. These airways are clearly defined, just like a road, and labeled on aeronautical charts. If the airway is below 18,000 feet, it is called a Victor (V) airway; above 18,000 feet, a Jet (J) airway. Navigating across the United States is quite similar to reading a road map. You start off at your home, and have to make lots of turns on smaller roads to reach a boulevard. The boulevard leads to the interstate, and you can finally drive a fairly straight course. In an airplane we do much the same. We fly departure routes, to the low altitude Victor airways, and then the Jet routes.

VOR’s send out radio signals in all directions. The more powerful VOR signals can be received up to 230 nautical miles, meaning two stations 460 miles apart can define an airway. The signal the VOR sends is unique for every degree of the compass. The aircraft’s on-board VOR receiver can accurately pinpoint where you are in relation to the station. This position from the station is called a radial. By using two VOR’s and determining exactly where the radials cross, you would know exactly where you are. Most stations also have a DME (Distance Measure Equipment) co-located at the VOR. By using one VOR to determine the radial from the station, and the DME to determine the distance, you can also pinpoint your position.

All airways are defined by specific radials. Flying from point A to point B, we fly outbound from one VOR on a specific radial (airway) to intercept the inbound radial to the next VOR. Cross the second VOR, and fly outbound on another predetermined radial, until a third VOR is within reception range. This continues until the aircraft reaches the arrival route to its destination.

During times of the day when ATC is not as busy, direct routing between more distant VOR’s can be approved. By eliminating the small zigs and zags of the airway system— no different than the small turns on the interstate—a few miles can be saved.

The next step in saving en route time, other than flying faster, is to fly the shortest distance between the departure and destination airport. This shortest distance is called the great circle route. Since the earth is round, the shortest distance between two points is a line that would appear as an arc on a flat map. Departing New York and flying east-northeast the entire flight, will take you to Paris. However, departing New York farther northeast bound, making an almost undetectable gradual turn to the east, and then flying southeast to arrive in Paris, is shorter by 127 miles. As a rule of thumb for east or westbound flights, the longer the flight the closer to the poles you will fly, to minimize the distance.

When flying a great circle route, special navigational equipment is required since the aircraft’s compass heading is constantly changing. An Inertial Reference System (IRS), and an Inertial Navigational System (INS) are the two most common long-range navigational systems in use today. Both these systems share the basic principal that if you enter into the computers exactly where you are while parked at the departure gate, where you want to fly, the great circle route will automatically be computed. Since these systems are completely independent of any ground- based radio signals, a minimum of three INS, or INS units, are required for cross-checking information and redundancy. The information computed by these systems can be displayed to the pilots on a conventional VOR left/right needle, or a cathode ray tube Flight Management System (FMS) found on the newer aircraft. In either case, to keep the airplane exactly on course, a constantly changing heading is necessary. On many flights, rhumb line (the one compass-course line) and great circle route courses are both utilized. Rhumb lines are used for the shorter segments and when ATC uses preplanned airways for traffic separation. Great circle routes are used on long hauls if the aircraft is so equipped.

AUTOPILOT

An autopilot is essentially a stabilizing system that’s capable of flying the airplane when the correct inputs are made. It relieves the pilots from the constant fine-tuning of the flight controls.

However, the autopilot is shrouded in a bit of a myth. Despite its complex computer system, the autopilot has no brain. It is capable of flying the airplane in climb, cruise, descent, approach, and sometimes landing, but only if the pilot tells it what to do. You can tell an autopilot to fly a specific heading and/or a specific altitude, and it will monitor itself to maintain those parameters. That’s it. On newer plans the autopilot does a magnificent job of flying nearly the entire flight-plan route—as long as it’s programmed to exact specifications. If there is bad weather ahead, a change in route or altitude, airspeed changes for traffic separation, the pilot must tell the autopilot a change is needed.

One essential convenience provided by the autopilot is its ability to constantly trim the plane along its longitudinal (pitch) axis. During flight, the aircraft’s center of gravity, it center of weight, is constantly changing in small increments as passengers move around, flight attendants wheel the galley carts up and down the aisles, and fuel is used. Pitch changes are constantly needed to maintain a steady state, whether that steady state be a constant climb or descent, or maintaining a level altitude. Once the autopilot is told what to do, it can make all those little changes without further pilot action.

ENGINE FAILURE DURING CLIMB, CRUISE, OR DESCENT

First, it is important to reemphasize how reliable turbo fan “jet” engines are. The chances of an engine malfunction are less than one per 50,000 hours of flight. But for the sake of example, let’s say an engine was to fail in cruise. With an engine shutdown, less total thrust is available, therefore the maximum cruising altitude will be lower. This lower maxi mum cruise altitude is called the drift down altitude. When dispatch is selecting a route, they must verify that the drift down altitude is at least 1000 feet above any terrain or obstacle. Typical drift down altitudes are 25,000 to 27,000 feet with one of two engines shut down on a twin, and two of four engines on a 747. If these altitudes seem higher than you would expect, remember, at cruise the power needed to maintain level flight is well below the power available. With one engine failed, the other can somewhat compensate.

If an engine falters in flight because of an internal part failure, there is little likelihood it can be restarted. However, if the failure was caused by a “flame-out” in the combustion section, or any other transient problem, a restart is possible. Pilots can differentiate between the two by consulting their engine windmilling charts. If the engine continues to windmill (spin) after a failure at greater than a calculated rate, the problem was not a part failure and a restart may be attempted. If a critical part has failed, the engine, in all likelihood, will not be spinning at all.

TOTAL ENGINE FAILURE

What if all the engines failed at once? It’s statistically improbable, but it has happened, and the result will surprise you. Recently, a Boeing 747 had all four engines fail simultaneously when flying through some high altitude volcanic ash. The ingestion of this heavy dust caused the engines to flame out.

Though a 747 is an extremely heavy airplane, weighing up to 850,000 pounds at takeoff, it is still capable of flying like a glider. From 35,000 feet with all engines failed, the airplane is capable of gliding approximately 70 miles in any direction to a suitable landing spot. How can something so big and so heavy glide? A 747 is a large airplane, but it also has a very large wing. The amount of weight each square foot of wing has to lift is no greater than aircraft many times smaller and lighter, so it can glide just as well.

To keep an airplane flying, air must pass over and under the wings. To keep this forward movement, you either need thrust or must fly slightly downhill. If your automobile engine quits driving downhill and you put the car in neutral, you will coast until the hill levels out. Same with a plane. In the case of this 747, they glided several minutes until they were clear of the volcanic dust that caused the problem, and restarted their engines. As a precaution, the crew landed at the nearest suitable airport for an inspection.

TOTAL HYDRAULIC FAILURE

Again, there are three or more completely redundant systems, making failure unlikely. But let’s take the highly improbable, and imagine that all the systems failed at once. In the smaller type aircraft, like the 727, the backup to a total hydraulic failure are basic cables. Though additional force is required, similar to losing your power steering in your car, the plane’s flight controls could be moved and normal flight maintained. On the larger aircraft, where the flight controls are quite a bit larger, a Ram Air Turbine (RAT) is installed as a backup to the multiple redundant hydraulic systems. With the loss of all hydraulics, the RAT would automatically drop from the lower fuselage, exposing a large propeller. As the plane traveled forward, the relative wind would spin the propeller, which in turn would spin a hydraulic pump, which would pressurize a reserve part of the hydraulics and allow the flight controls to be moved. Further, if the RAT did not deploy automatically, it could be lowered manually.

AIR-CONDITIONING SYSTEM FAILURE

If all your air-conditioning systems were to fail, which is highly unlikely, the outflow valve would automatically close tight. With all the air-conditioning systems failed, no new compressed and conditioned air would be entering the cabin. Since the outflow valve was also fully closed, almost no air would be escaping, so there would be plenty of breathable air.

Gradually it would get stuffy, but an emergency descent would not be required, though priority ATC handling would be requested. There would be plenty of time for a gradual descent to a lower altitude where the fresh outside breathable air could be filtered through the cabin. In all likelihood, the overhead emergency oxygen masks would never be needed, though most flight crews would deploy them to alleviate concern.

PRESSURIZATION SYSTEM FAILURE

If the entire pressurization system was to fail simultaneously—all three independent pressurization controllers at once—a rapid depressurization might not result either. Why? Without any pressurization signals, the out flow valve will independently close at a predetermined set cabin altitude, usually 11,000 feet. Then you’re back to a similar condition as described above.

To get a rapid depressurization as depicted in the movies, all the outflow valve motors—both A.C. and D.C.—must fail at once with the outflow valve in the open position. A bomb puncturing the fuselage would also create a large enough hole to do the same. In such a depressurization, because of the higher pressure in the plane, inside air rushes Out quickly, equalizing with the lower-pressure outside air. The equalization of pressure is over almost instantaneously. The outflow valve is located behind and below the cargo compartment, so it would be impossible to lose someone that way. During an explosion, a passenger in the immediate vicinity may not be as fortunate, but if you survive the first few seconds, stay calm, you will survive.

Airplanes are required to be able to withstand a 20- square-foot hole blown in the fuselage without the loss of structural integrity. The manufacturers routinely double that standard to 40 square feet.

The first sign of a rapid depressurization would be a loud noise. The mixing of the cold outside air with the warmer cabin air inside will cause an immediate fogging of the plane. It will clear within minutes. The plane will get chilly and your ears may pop or become blocked. The oxygen masks will drop instantly, and it is your responsibility to pull it toward you, starting the oxygen flow. If you are traveling with a child, put your own mask on first.

The cockpit crew all have special quick-donning oxygen masks that can be put on with just one hand. Within two seconds the cockpit crew will be on 100 percent oxygen, alleviating the danger of losing any cognitive abilities from lack of oxygen. In routine flight above 25,000 feet, if one pilot leaves his station, for whatever reason, the other must go on oxygen. Above 41,000 feet, as a precaution, one pilot must be on oxygen all the time.

In a rapid depressurization, after the flight crew dons their oxygen masks they will assess the situation. The most rapid emergency descent may not be warranted if there is concern for the plane’s structure. An emergency will be declared with Air Traffic Control, which will provide a priority clearance to a lower altitude. Again, it may not be prudent simply to descend; verification that the altitudes below are clear of other aircraft is worth a few seconds of delay. The power will be retarded to idle. The captain will lower the aircraft’s pitch attitude to between 5 and 10 degrees nose down. Doesn’t sound like a great deal, but it will feel very steep. The speed brakes will be deployed to compensate for the tendency of the airplane to accelerate. The descent rate will be greater than 6000 feet per minute, the maximum we can read on our vertical speed indicators. Level off will be at 14,000 feet or below, low enough so supplemental oxygen is no longer needed. The emergency descent will be over in about three minutes, considerably less than the minimum 12 minutes of oxygen carried on board. Pilot’s practice this whole procedure during recur rent training every six months.

DESCENT

Imagine there is no Air Traffic Control, no conflicting air traffic, no wind or inclement weather. If descent was taking place in a completely uncluttered environment—one free of Air Traffic Control, other planes, wind and weather— pilots would descend on a three to one line. For every three miles of distance traveled toward the destination, 1000 feet of altitude would be lost. If the plane were cruising at 35,000 feet, an ideal descent would be started no later than 105 miles from the airport. Since it is highly desirable to be smooth, an additional 15 miles would be added to the 105 miles, so a slow transition could be made from cruise to descent.

In reality, though, there are many other factors to consider. Tailwinds might make it necessary to start the descent earlier to compensate for the increased speed over the ground. Headwinds might require a later top of descent point, or a more gradual rate. ATC altitude restrictions might dictate a descent in stages, step-down clearances similar to the step climbs on the way up, with the resultant pitch and power changes. The flight may have to fly slightly out of its way to be sequenced with other arrival traffic. Flying beyond the airport and circling back to land into the wind can add 40 miles to a flight, another necessary part of the calculation.

But in general terms, most flights plan to arrive 30 miles from the destination—at 10,000 feet and already slowed to 250 knots (288 mph)—which is ideal in making a smooth transition to the approach segment.

Given ATC clearance at the calculated top of descent point, the flight crew begins a very gradual pitch down of the nose—1 or 2 degrees lower than the level cruise attitude—and the plane begins to descend. Initially, the rate of descent is only 1000 feet per minute. After four minutes—or 4000 feet, as a general rule—the descent rate will be increased. The nose of the plane will be lowered a couple of degrees further and the rate of descent increased to 2000 to 3000 feet per minute. The airplane is capable of descent rates significantly higher, and at times, because of conflicting traffic and late ATC descent clearances, higher rates are used. When ATC clears an aircraft out of its cruising altitude—usually to 24,000 feet first, and later 10,000 feet—traffic separation must be provided at every altitude the plane descends through. It would be a consider ably more difficult task if it were not for the SID’s and STAR’s mentioned earlier. Standard Instrument Departures, and Standard Terminal Arrival Routes group, sequence and separate the inbound traffic from the outbound.

There are times when Air Traffic Control asks a plane to descend and slow down at the same time. Airplanes are designed with very little aerodynamic drag, so even with the power back to idle, it can be difficult to slow in a descent. For this purpose, speed brakes, those large panels near the trailing edge of the wing, were installed. Normally the panels are completely stowed in the zero-degree position. With a pull of the speed brake handle in the cockpit, they can be raised in any increment from zero to 40 degrees. The extra drag serves to slow the aircraft down while increasing the descent rate without further lowering the nose of the plane.

One final note about descent. Given no descent restrictions, it would be possible from just beyond the top of descent point to reduce power on all the engines to idle and glide to the destination. In fact, the most fuel-efficient descent safely incorporates idle power glides into each and every flight.

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