The Flight: Approach to Land: Airplane and Airline FAQ

RUNWAYS

Landing requires less distance than takeoff, so it’s possible to land at an airport that you can’t later depart from. Most runways in the United States are 8000 to 10,000 feet long—almost two miles. With a maximum effort, most planes could be stopped in 2500 feet, but that would be uncomfortable and cause a great deal of brake and tire wear. Airlines use approximately 5500 feet as a minimum.

In cruise—or in the early part of the descent—planning for approach begins. Similar to takeoff, a thorough review must be made to verify that the flight crew, airplane, and airport all meet the minimum standards to begin the approach.

For the first 100 to 300 hours, new captains—either first-time captains or just new to an airplane—must ob serve higher than standard approach minimums. This term, “minimums,” means the lowest altitude a pilot can descend without having visual contact with the runway. On average, minimum altitudes are raised 100 feet and the minimum visibility requirement one-half mile for the pilot new in the “left seat.” The fact that the captain may have flown thousands of hours as a co-pilot on this very plane does not count. High minimums still apply.

The airplane must also be legal. If something has mal functioned in flight, depending on the type of problem, better weather will be required before the plane is cleared to approach at the lowest minimums. The runway has to be long enough. Of course, you will never be dispatched to an airport with a runway too short to accommodate your flight. A runway is considered long enough if the plane can come to a complete stop in 60 percent of the total length after touching down 1000 feet from the runway threshold (beginning).

What if during flight one of the brakes or antiskid systems developed a problem? Naturally, the stopping capability of the plane is reduced, which would require a longer runway. Consulting an Aircraft Restrictions Manual (ARM), which is carried in every cockpit, the pilot can discern if the 40 percent margin of error is still met. The use of reverse thrust after landing is never calculated into the landing figures.

RUNWAY CONDITIONS

Stopping capability can also be affected by the condition of the runway itself. As noted earlier, runways are grooved and crowned—grooved to provide rapid drainage, and crowned in the center to direct water off to the sides. Still, wet runways can be slippery. Whenever the runway is reported wet, or the visibility goes below three-quarters of a mile—there’s moisture in the air— 15 percent has to be added to the minimum runway length. Above one-half inch of water on the runway and a landing would be illegal. Snow on the runway also requires extra approach planning. Wet slushy snow has similar limits as standing water. Dry snow provides more friction for stopping than wet snow, so a slightly greater accumulation is allowed. However, if the stopping ability ever gets below a certain level as deter-. mined by an airport vehicle designed to measure braking coefficients, or as reported by a flight crew, the runway has to be plowed. Runways are routinely plowed long before they ever get to that point.

Other situations can also cause a runway to be too short to land. If another plane becomes disabled on the runway, it will have to be closed. If maintenance is required, then that portion of the runway will have to be closed.

AIRPORT WEATHER

Near the top of descent the pilots listen to the arrival ATIS (Automatic Terminal Information Service). The arrival ATIS provides weather for the current destination and the active runways. It’s updated every hour, or as conditions change. Included in the broadcast are:

1. Cloud coverage and how low to the ground they are, so pilots know what to expect when they “break out”

2. Visibility, so they know how far they should be able to see when they do “break out”

3. The kind and extent of any precipitation, so pilots know the reason for the visibility restrictions

4. The temperature (Could the precipitation freeze? Do they need engine and wing anti-ice?)

5. The dewpoint, in order to discern the possibility of fog

6. The wind direction and velocity—calm or gusty, straight down the runway or a crosswind

7. The barometric altimeter setting, to recalibrate altimeters for current height above sea level

8. The active runways and the type of instrument procedure used to navigate to that runway

9. Any special situations, including the field conditions, braking action, RVR (runway visual range), the actual visibility as measured by a machine (transmissometer) located at the runway, forecast or reported windshear, runway cutback, taxiway closed, lights inoperative, or anything else pertinent

APPROACH PLANNING

Among the items on the ATIS tape is a description of the active runway and the instrument approach available to use. Many runways have more than one type of electronic guidance system to align you with the runway when visibility is poor. All pilot flight kits contain a complete set of manuals depicting the approach procedure for every air port served by that airline, plus all the alternate airports. Each approach chart for the airport is labeled by name, numbered, and dated. Once the type of approach and runway have been assigned, a thorough review of the procedure takes place.

First you verify the correct approach plate. With numerous parallel runways, it would be a grand mistake to confuse 27R (right) with 27L (left). Each approach has a specific navigational aid with its own distinct frequency which must be tuned and identified. Morse code transmit ted over this special frequency is still used for positive identification. The inbound magnetic course must be set. For example, a runway named Runway 27 faces roughly 270 degrees, which is west on the compass. Since runway names are rounded to the nearest 10 degrees, Runway 27’s direction could be anywhere from 265 to 275 degrees. However, on an approach, we want to be accurate to the nearest degree, so the approach chart tells us the exact magnetic direction of the runway. If it’s 272 degrees, we set 272.

Crossing altitudes must be reviewed. The approach chart tells you exactly what altitude you should be at any specific point. Also the lowest minimum altitude you can descend, without visual reference to the runway, must be determined. The approach chart highlights the standard mini mum altitudes with everything operating, as well as more restrictive higher minimum altitudes if any of the ground- based equipment or airport lighting is not operating. The pilots determine which minimums apply.

Just like almost everything else, pilots always plan for “what if” situations. The “missed approach procedure” is for the rare time when you don’t have visual contact with the runway at the decision height and must go-around. One hundred feet off the ground with no forward visibility is no time to be deciding which way to fly. The approach chart also highlights any special terrain features and so will guide you away from any menacing terrain. Finally, after the approach briefing is completed by the pilots, a written approach checklist is read to verify that all items have been completed.

APPROACH SEQUENCING

Through the approach preparation and briefing, the plane is still flying toward the airport. The actual airspeed of the plane may be slowing down, but the pace of the pilot’s job is increasing. Descending through 10,000 feet, the ATC speed limit must be observed; the plane will be slowed to 250 knots. The Sterile Cockpit Rule applies as it did on initial climb, so only announcements relating to the safety of flight can be made by the pilots. No more sightseeing announcements.

At busy airports it’s rare to be cleared straight into the runway. Some form of approach sequencing usually takes place. Initially, ATC will request a speed adjustment to increase the spacing between inbound traffic. The lead airplanes fly faster and the trailers slow down. Since approach control has to sequence traffic from all directions, not just the arrival route you are flying, additional spacing may be required.

This is where vectors are used. Vectors are turns issued by ATC off the published arrival route. A couple of turns left and right may adjust the traffic pattern perfectly. However, if ATC becomes saturated with inbound traffic and has no airspace closer to the airport available for your flight, a holding will be issued. A holding pattern is essentially an oval orbit in the sky. If a holding is issued, the plane flies to a clearance limit point, called the holding fix, and makes right or left turns in the sky. The pattern is oval so not as many turns have to be made. When weather closes an airport temporarily, planes sometimes get “stacked up” in a holding pattern. What this means is that more than one airplane is holding at the same fix, but never at altitudes closer than 1000 feet.

During approach delays, a satisfying announcement to passengers is hard to make. Pilots are acutely aware that many passengers have connecting flights to catch. They are also sensitive that the approach Air Traffic Controller is very busy and does not have time to talk with the other approach controller working the other inbound sectors to determine your exact sequence. Approach does issue an “expect further clearance” or “expect approach clearance” time, but that can change. Delays are a nuisance, an inconvenience to the passenger, and an added expense to the airline. But safety can make them unavoidable.

INSTRUMENT LANDING SYSTEM APPROACH (ILS)

The primary approach at most airports uses the Instrument Landing System. An ILS transmitter, located at the runway, sends signals that are received by the multiple ILS receivers on board. Actually, it sends two separate and distinct signals. A localizer signal is transmitted in a narrow horizontal beam 6 degrees wide, and lets you know if you are right, left, or directly on course. A glide slope beam 1.4 degrees vertical tells you if you are on the correct slope to the runway. Normal glide slopes are angles at 3 degrees, requiring 300 feet of altitude to be lost every mile on the approach, or approximately 700 feet per minute. Not very steep.

Two needles are displayed on the flight instruments. If the localizer needle is exactly centered, the airplane will be directly in line with the runway center line. Any time the needle moves from that perfect center position, a slight turn is needed to correct back to course. There are dots on the localizer scale. Each dot on the instrument represents one degree. If the needle moves one dot, you’re one degree off. More than 2 degrees will cause full-scale needle deflection. One-half dot off course, with weather near minimums, is reason enough to initiate a go-around, even though one-half dot is only 175 feet off course.

The glide slope is like the localizer, but turned on its side and four times more precise. Being just 14 feet high or low over the runway will cause a full-scale needle deflection. Part of its precision is due to the location of the transmitters. The glide slope transmitter is only 1000 feet beyond the runway threshold. Because it’s providing only vertical guidance, it does not have to be on the runway—only aligned with the touchdown aim point. The localizer antenna, on the other hand, must be perfectly aligned with the runway center line, which is why it’s located at the far end of the runway—up to two miles away.

A complete ILS system has many other necessary components, including altitude marker beacons, lights, and run way markings. A high intensity approach lighting system begins more than a half mile from the threshold. Sequence flashers and white strobe lights that flash in rapid-fire succession toward the runway provide visual lead-in guidance for runway alignment. These sequence flashers arc commonly called “the rabbit,” because of the hop, hop, hop visual effect. Runway end identifier lights—which actually mark the beginning—runway edge lights, runway center line lights, and runway touchdown zone lights, all highlight the exact dimensions of the runway.

The touchdown aim point is 1000 feet from the thresh old. If you aimed for the very beginning of the runway, there would be no margin of error for landing short. A 3000-foot touchdown zone is highlighted with lights and big heavy white lines painted on the cement. If a touch down can’t be made in the touchdown zone, a go-around will be made. Runway remaining lights use colors to indicate how much runway is left. At 3000 feet, white and red; at 2000 feet it’s amber; the final 1000 feet is all red.

DIFFERENT GRADES OF INSTRUMENT LANDING SYSTEM (ILS) APPROACHES

ILS approaches are divided into three categories: CAT I, CAT II, and CAT III. CAT I approaches allow a descent to a minimum-decision-height altitude of 200 feet above ground level and a landing with 1800 feet forward visibility as measured by the RVR (runway visual range) meter. CAT II minimums are a decision height of 100 feet AGL and 1200 feet RVR. CAT III is the epitome of a high technology approach. No decision height is stipulated because a full autopilot-autoland system is required and used to land the plane. Visibility is required to be 700 feet RVR (soon to be lowered to 300 feet), but very little if any of the runway will be seen by the pilots before touchdown. Why the difference in visibility? The airport itself must adhere to higher standards to receive and maintain CAT II and CAT III certification. For the airlines to use this capability, their airplanes must be equipped and maintained to higher standards, and their pilots specifically trained in these extremely low-visibility landing procedures. What this means to the passengers is that some pilots and planes can land in denser fog than others.

VISUAL APPROACH

The other common instrument approach is a visual approach. Using instrument and visual together may seem like a contradiction, but a visual approach is an instrument procedure. Radar coverage, electronic approach guidance, and full localizer and glide slope information are all still used. But when the visibility is good, pilots can also see the runway and other aircraft from long distances. Spacing between airplanes can be safely decreased using “visuals.” For example, on parallel runways closer together than 4300 feet, aircraft cannot fly the approach side by side when the visibility is restricted. When the visibility is greater than three miles, this restriction no longer applies.

Visual approaches are also used for noise abatement procedures. Landing to the south at Washington National, for example, a Potomac River noise abatement visual is common. By flying the entire approach over the river, noise from the aircraft can be minimized on the ground. Pitch and power changes will be frequent, and multiple turns will have to be made at low altitude. LaGuardia Airport in New York has a similar noise abatement visual approach procedure. Arriving from the south, the pilots are required to fly over the Long Island Expressway until turning for Runway 31. San Francisco has the Quiet Bridge and Tipp Toe visual approaches. Flying over the San Francisco Bay until run way alignment is needed for landing is a good-neighbor policy. Other airports have similar noise abatement procedures.

Weather permitting, some airports use visual approaches as part of the “keep them high” noise abatement policy. Aircraft regularly are required to maintain altitudes above the normal 3 degree descent path. Once given approach clearance, a steeper than normal descent rate will be used to re-intercept the normal slope. The intercept will always occur a safe distance from the runway. The higher altitude limits the noise that can be heard on the ground. The steeper rate reduces the power requirement. Visual approaches, with or without noise abatement procedures, are never issued when weather is a factor.

FLYING THE APPROACH

Within 10 to 20 miles of the airport the aircraft will gradually be slowed to the minimum flaps up “clean speed”—usually 210 to 230 knots (242 to 265 mph). The flight attendants will be signaled that the plane is five to ten minutes from landing, which gives them time to complete their FAA-required duties and be seated themselves. All seat backs must be upright, the tray tables and any carry-on luggage properly stowed, and if a smoking flight, all cigarettes extinguished. Entering the airport traffic pattern downwind, the flaps and slats will begin to be lowered in increments. For example, on the 767 the flaps have flap settings of 1, 5, 15, 20, 25, and 30 degrees. The slats are extended as a function of flap position. Flap and slat activation can be noisy. With the flaps and slats partially or fully extended, a vibration can also be felt from the additional aerodynamic drag.

Each flap position has a maximum and minimum speed. The maximum speed is fixed, a never-exceed structural limit speed for each degree of extension. The minimum speeds for each position are calculated by the flight crew to always provide the 30 percent margin above stall speed. Typical minimum speeds are 180 knots (207 mph) for flaps 5 degrees; 160 knots (184 mph) for flaps 15; 140 knots (161 mph) for flaps 20; and a final approach speed for full flaps of 130 to 135 knots (150 to 155 mph). These are the minimums. Many times the airspeeds will be higher. The first several flap positions, combined with slat extension, do the most to increase the lifting capability of the wing. Additional flap extension predominately increases the drag.

Approximately five to seven miles from the runway, the landing gear is lowered. Similar to takeoff, after the landing gear goes down, the gear doors are closed again. The landing gear is a very high-drag item. An earlier than normal extension can help to increase the descent rate without increasing the airspeed. The gear will be down and locked a minimum of three miles from touchdown, 1000 feet above the ground. The flaps will be in the landing position, the airspeed slowed to the final approach speed, and the before-landing checklist completed to verify every thing is done.

WAKE TURBULENCE

After a smooth flight, it’s disappointing to have less than a perfectly smooth approach. However, just like the takeoff and initial climb, the plane on approach is back to a lower-altitude environment, where convective air can cause gusty winds and low-level chop. Also, with flaps extended the outboard ailerons—most larger planes have two sets— are active, and an increase in the roll sensitivity can sometimes be felt. Also, the flying has to be more precise on approach. Being a few feet off in cruise is insignificant. But now the pilots are constantly making small adjustments to stay exactly on course. Most of these are expected and understood by the passengers.

What usually gives rise to most concern during approach is wake turbulence—also called wing-tip vortices. As discussed in aerodynamics, the high pressure below the wing and the low pressure on top is the fundamental reason for lift. Around the wing tip the only place where the air can swirl from the bottom to the top is where the two differential pressures try to equalize. This circular motion can develop tremendous speed and temporarily upset the air even on a smooth, calm day. Think of the motorboat analogy. Take a boat on a lake that’s perfectly calm, and as you speed through the water, a sizable wake is created. After a few moments the water returns to its original calm.

Wake turbulence created by the wings of the airplane is similar, except that pilots can’t see it. Since the wake of a plane has a tendency to descend as well as dissipate rapidly, being behind another aircraft at the same altitude should not cause a bumpy ride. Approach control spaces aircraft so they will never fly too close to the wake of the plane in front. Normal separation is a minimum of three miles, which is Increased to a minimum of five miles if you are following a heavy jumbo jet.

However, wake turbulence can be blown laterally in a crosswind as it is descending. There are times, no matter the extent of the precautions, when you will inadvertently fly through another aircraft’s wake turbulence. The circular motion of the wake causes a rolling motion of the plane. It’s felt as a sudden jolt and is over almost instantly. Wake turbulence is usually unexplained by the flight crew because of the Sterile Cockpit Rule.

WINDSHEAR

Windshear is a rapid change of wind speed and/or direction over a very short distance. Windshear can be found at any altitude, but only near the ground does it become a concern. A vertical windshear, called a microburst, is the most hazardous because the downdraft can be severe. Picture a bucket of water being poured onto your driveway. The water falls straight down, but upon hitting the ground, much of it splashes outward and up ward.

What happens? When you approach the runway, you first encounter the updraft—the water from the bucket being splashed back up—and the airplane has a tendency to rise. The indicated airspeed also increases because of the rapidly increasing headwind—the water being splashed to the side. To an unsuspecting pilot, the tendency would be to lower the nose to reestablish the glide slope and to reduce the power to slow down. A split second later, you fly into the core of the downdraft—the water being poured from the bucket. The airspeed drops and the rate of descent in creases. The power must be rapidly reapplied and a climb initiated to counter the effect of the downburst. Like the bucket of water, the column of descending air is very small, isolated and short-lived. Moments after encountering a downburst, the incident will be over.

Avoidance is the safest way of dealing with a microburst. Major airports have Low Level Windshear Alert Systems (LLWAS) which measure the wind direction and velocity at numerous locations near the airport. Conflicting readings at different locations can indicate a shear, and the tower will issue a warning. Currently, this LLWAS is being expanded to include more wind reporting locations farther outside the airport boundary. Since microbursts can be very isolated, off-airport sites are essential. A new Doppler Radar system that can locate and measure the intensity of windshear and microbursts is also being tested.

For sake of example, take a worst-case scenario. No suspicious weather, and no ground-based warnings. A plane is established on its final approach, stabilized, and descending through 1000 feet. Any time the flight path becomes somewhat unstable—the airspeed fluctuates plus or minus 15 knots, the vertical speed changes plus or minus 500 feet per minute, the pitch attitude goes up or down 5 degrees, an unusual amount of power is required to fly the approach, or you are off the glide slope by one degree—a microburst must be suspected and a windshear recovery initiated. Airplanes are capable of climb rates of 3000 and 4000 feet a minute, so the quicker the recovery is begun, the better.

New and in-service aircraft are now being fitted with a new windshear detection system. When the onboard windshear computer senses a windshear, an alert—both visually on the flight instruments and aurally using a computer-generated voice saying “Windshear, windshear”

—will alarm, and windshear recovery will be started immediately.

NAVIGATION EQUIPMENT FAILURE AND NO-GYRO APPROACHES

If during the flight all navigation equipment happened to fail—the multiple ILS’s, multiple VOR’s, multiple NDB’s, everything—the logical alternative is to proceed to an airport where the weather is good. ATC can give radar vectors to whatever airport is determined to be the best alternative. If the weather is bad at all the surrounding airfields, and an instrument approach is necessary to find the runway through the clouds, options still exist. If all your flight instruments are working and the problem is only one of navigation, an ASR (Airport Surveillance Radar), or a PAR (Precision Approach Radar), can be used. At civilian airports, approach control can guide you nearly all the way to the runway with precise radar vectors. At military airports, always available to commercial aircraft in an emergency, a PAR approach is available. A PAR approach is the military equivalent of an ILS, but no airborne equipment is required. A military ATC specialist can give you extremely accurate headings—within one degree— and glide slope guidance to the runway by using his specialized radar.

Should all flight instruments—the captain’s, co-pilot’s, and all the standby be inaccurate—a no-gyroscope approach can be made. Instead of issuing specific headings, the radar controller commands “turn, and stop turn.” Rudimentary, but available as an emergency alternative.

CAN’T GET THE LANDING GEAR DOWN

Landing gear that can’t be lowered is of great concern to many passengers. Hydraulic pressure is used to raise the landing gear into their respective wheel wells. Once completely raised and the gear doors shut, the gear is mechanically locked in the up position. On some aircraft the gear itself is locked, on others the gear simply rests on the locked gear door. After being mechanically locked, the hydraulic pressure is no longer needed and turned off.

Hydraulic pressure is normally used to lower the gear back into the down and locked position. But suppose the primary system fails to operate? The landing gear is quite heavy and massive. If you were able to release the up locks, gravity would do the rest of the job for you. Installed on all aircraft are secondary landing-gear extension systems that use either cables or electrical motors to release those locks. But if all the gear still failed to lock in the down position, there are flight maneuvers that can be used to force them down.

In the worst-case scenario, you can safely land on the usable gear, or with the gear completely up, with very little damage to the airplane—and more important, with no injuries. Airplanes that have to land with one or more gear failed have been repaired, inspected, and returned to service in less than 48 hours.

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