Navigation in the Arrival Environment

Even in a radar environment, precise navigation and position reports, when required, are still a primary means of providing separation. In most situations, ATC does not have the capability or the responsibility for navigating an aircraft. Because they rely on precise navigation by the flight crew, flight safety in all IFR operations depends directly on the pilot’s ability to achieve and maintain certain levels of navigational performance. ATC uses radar to monitor navigational performance, detect possible navigational errors, and expedite traffic flow. In a non-radar environment, ATC has no independent knowledge of the actual position of the aircraft or its relationship to other aircraft in adjacent airspace. Therefore, ATC’s ability to detect a navigational error and resolve collision hazards is seriously degraded when a deviation from a clearance occurs.

The concept of navigation performance, previously discussed in this site, involves the precision that must be maintained for both the assigned route and altitude. Required levels of navigation performance vary from area to area depending on traffic density and complexity of the routes flown. The level of navigation performance must be more precise in domestic airspace than in oceanic and remote land areas since air traffic density in domestic airspace is much greater. For example, there are three million flight operations conducted within Chicago Center’s airspace each year. The minimum lateral distance permitted between co-altitude aircraft in Chicago Center’s airspace is eight nautical miles (NM) (3 NM when radar is used). The route ATC assigns an aircraft has protected airspace on both sides of the centerline, equal to one-half of the lateral separation minimum standard. For example, the overall level of lateral navigation performance necessary for flight safety must be better than 4 NM in Center airspace. When STARs are reviewed subsequently in this section, it is demonstrated how the navigational requirements become more restrictive in the arrival phase of flight where air traffic density increases and procedural design and obstacle clearance become more limiting.

The concept of navigational performance is fundamental to the code of federal regulations and is best defined in Title 14 of the Code of Federal Regulations (14 CFR) Part 121, § 121.103 and 121.121, which state that each aircraft must be navigated to the degree of accuracy required for ATC. The requirements of 14 CFR Part 91, § 91.123 related to compliance with ATC clearances and instructions also reflect this fundamental concept. Commercial operators must comply with their Operations Specifications (OpSpecs) and understand the categories of navigational operations and be able to navigate to the degree of accuracy required for the control of air traffic.

In the broad concept of air navigation, there are two major categories of navigational operations consisting of Class I navigation and Class II navigation. Class I navigation is any en route flight operation conducted in controlled or uncontrolled airspace that is entirely within operational service volumes of International Civil Aviation Organization (ICAO) standard navigational aids (NAVAIDs) (very high frequency (VHF) omnidirectional radio range (VOR), VOR/ distance measuring equipment (DME), non-directional beacon (NDB), etc.).

Class II navigation is any en route operation that is not categorized as Class I navigation and includes any operation or portion of an operation that takes place outside the operational service volumes of ICAO standard NAVAIDs. For example, aircraft equipped only with VORs conducts Class II navigation when the flight operates in an area outside the operational service volumes of federal VORs. Class II navigation does not automatically require the use of long-range, specialized navigational systems if special navigational techniques are used to supplement conventional NAVAIDs. Class II navigation includes transoceanic operations and operations in desolate and remote land areas, such as the Arctic. The primary types of specialized navigational systems approved for Class II operations include inertial navigation system (INS), Doppler, and global positioning system (GPS). Figure 1 provides several examples of Class I and II navigation.

Planning the descent from cruise is important because of the need to dissipate altitude and airspeed in order to arrive at the approach gate properly configured. Descending early results in more flight at low altitudes with increased fuel consumption, and starting down late results in problems controlling both airspeed and descent rates on the approach. Prior to flight, pilots need to calculate the fuel, time, and distance required to descend from the cruising altitude to the approach gate altitude for the specific instrument approach at the destination airport. While in flight prior to the descent, it is important for pilots to verify landing weather to include winds at their intended destination. Inclimate weather at the destination airport can cause slower descents and missed approaches that require a sufficient amount of fuel that should be calculated prior to starting the descent. In order to plan the descent, the pilot needs to know the cruise altitude, approach gate altitude or initial approach fix altitude, descent groundspeed, and descent rate. This information must be updated while in flight for changes in altitude, weather, and wind. The approach gate is an imaginary point used by ATC to vector aircraft to the final approach course. The approach gate is established along the final approach course 1 NM from the final approach fix (FAF) on the side away from the airport and is located no closer than 5 NM from the landing threshold. Flight manuals or operating handbooks may also contain a fuel, time, and distance to descend chart that contains the same information.

One technique that is often used is the descent rule of thumb, which is used to determine when you need to descend in terms of the number of miles prior to the point at which you desire to arrive at your new altitude. First, divide the altitude needed to be lost by 300. For example, if cruising altitude is 7,000 feet and you want to get down to a pattern altitude of 1,000 feet. The altitude you want to lose is 6,000 feet, which when divided by 300 results in 20. Therefore, you need to start your descent 20 NM out and leave some extra room so that you are at pattern altitude prior to the proper entry. It is also necessary to know what rate-of-descent (ROD) to use.

To determine ROD for a three-degree path, simply multiply your groundspeed by 5. If you are going 120 knots, your ROD to fly the desired path would be 600 fpm (120 × 5 = 600). It was determined in the previous example that a descent should be initiated at 20 NM to lose 6,000 feet. If the groundspeed is 120 knots, that means the aircraft is moving along at 2 NM per minute. So to go 20 NM, it takes 10 minutes. Ten minutes at 600 fpm means you will lose 6,000 feet.

The calculations should be made before the flight and rules of thumb updates should be applied in flight. For example, from the charted STAR pilots might plan a descent based on an expected clearance to “cross 40 DME West of Brown VOR at 6,000” and then apply rules of thumb for slowing down from 250 knots. These might include planning airspeed at 25 NM from the runway threshold to be 250 knots, 200 knots at 20 NM, and 150 knots at 15 NM until gear and flap speeds are reached, never to fall below approach speed.

Vertical navigation (VNAV) is the vertical component of the flight plan. This approach path is computed from the top-of- descent (TOD) point down to the end-of-descent waypoint (E/D), which is generally the runway or missed approach point, which is slightly different than to the approach gate for non-flight management system (FMS) equipped aircraft. [Figure 2] The VNAV path is computed based upon the aircraft performance, approach constraints, weather data (winds, temperature, icing conditions, etc.) and aircraft weight.

The two types of VNAV paths that the FMS use is either a performance path or a geometric path. The performance path is computed using at idle or near idle power from the TOD to the first constrained waypoint. [Figure 3] The geometric path is computed from point to point between two constrained waypoints or when on an assigned vertical angle. The geometric path is shallower than the performance path and is typically a non-idle path. [Figure 4]

Lateral navigation/vertical navigation (LNAV/VNAV) equipment is similar to an instrument landing system (ILS) in that it provides both lateral and vertical approach course guidance. Since precise vertical position information is beyond the current capabilities of the GPS, approaches with LNAV/VNAV minimums make use of certified barometric VNAV (baro-VNAV) systems for vertical guidance and/or the wide area augmentation system (WAAS) to improve GPS accuracy for this purpose.

Note: WAAS makes use of a collection of ground stations that are used to detect and correct inaccuracies in the position information derived from the GPS. Using WAAS, the accuracy of vertical position information is increased to within three meters.

To make use of WAAS; however, the aircraft must be equipped with an IFR-approved GPS receiver with WAAS signal reception that integrates WAAS error correction signals into its position determining processing. The WAAS enabled GPS receiver [Figure 5] allows the pilot to load an RNAV approach and receive guidance along the lateral and vertical profile shown on the approach chart. [Figure 6] It is very important to know what kind of equipment is installed in an aircraft, and what it is approved to do. It is also important to understand that the VNAV function of non-WAAS capable or non-WAAS equipped IFR-approved GPS receivers does not make the aircraft capable of flying approaches to LNAV/VNAV minimums.

FMS are the primary tool for most modern aircraft, air carriers, and any operators requiring performance based navigation. Most of the modern FMS are fully equipped with LNAV/VNAV and WAAS. The FMS provides flight control steering and thrust guidance along the VNAV path. Some less integrated systems may only advise the flight crew of the VNAV path but have no auto-throttle capability. These less integrated systems require an increase in pilot workload during the arrival/approach phase in order to maintain the descent path.

The need to plan the IFR descent into the approach gate and airport environment during the preflight planning stage of flight is particularly important for turbojets. TOD from the en route phase of flight for high performance aircraft is often used in this process and is calculated manually or automatically through a FMS based upon the altitude of the approach gate. A general rule of thumb for initial IFR descent planning in jets is the 3 to 1 formula. This means that it takes 3 NM to descend 1,000 feet. If an airplane is at FL 310 and the approach gate or initial approach fix is at 6,000 feet, the initial descent requirement equals 25,000 feet (31,000–6,000). Multiplying 25 times 3 equals 75; therefore, begin descent 75 NM from the approach gate, based on a normal jet airplane, idle thrust, speed Mach 0.74 to 0.78, and vertical speed of 1,800–2,200 fpm. For a tailwind adjustment, add 2 NM for each 10 knots of tailwind. For a headwind adjustment, subtract 2 NM for each 10 knots of headwind. During the descent planning stage, try to determine which runway is in use at the destination airport, either by reading the latest aviation routine weather report (METAR) or checking the automatic terminal information service (ATIS) information. There can be big differences in distances depending on the active runway and STAR. The objective is to determine the most economical point for descent.

An example of a typical jet descent-planning chart is depicted in Figure 7. Item 1 is the pressure altitude from which the descent begins; item 2 is the time required for the descent in minutes; item 3 is the amount of fuel consumed in pounds during descent to sea level; and item 4 is the distance covered in NM. Item 5 shows that the chart is based on a Mach .80 airspeed until 280 knots indicated airspeed (KIAS) is obtained. The 250 knot airspeed limitation below 10,000 feet MSL is not included on the chart, since its effect is minimal. Also, the effect of temperature or weight variation is negligible and is therefore omitted.

Due to the increased flight deck workload, pilots should get as much done ahead of time as possible. As with the climb and cruise phases of flight, aircrews should consult the proper performance charts to compute their fuel requirements, as well as the time and distance needed for their descent.

During the cruise and descent phases of flight, pilots need to monitor and manage the aircraft according to the appropriate manufacturer’s recommendations. Flight manuals and operating handbooks contain cruise and descent checklists, performance charts for specific cruise configurations, and descent charts that provide information regarding the fuel, time, and distance required to descend. Aircrews should review this information prior to the departure of every flight so they have an understanding of how the aircraft is supposed to perform at cruise and during descent. A stabilized descent constitutes a preplanned maneuver in which the power is properly set, and minimum control input is required to maintain the appropriate descent path. Excessive corrections or control inputs indicate the descent was improperly planned. Plan the IFR descent from cruising altitude so that the aircraft arrives at the approach gate altitude or initial approach fix altitude prior to beginning the instrument approach. For example, suppose you are asked to descend from 11,000 feet to meet a crossing restriction at 3,000 feet. [Figure 8] Since there is a 200 knot speed restriction while approaching the destination airport, you choose a descent speed of 190 knots and a descent rate of 1,000 fpm. Assuming a 10 knot headwind component, groundspeed in the descent is 180 knots.

Making the transition from cruise flight to the beginning of an instrument approach procedure sometimes requires arriving at a given waypoint at an assigned altitude. When this requirement is prescribed by a published arrival procedure or issued by ATC, it is called a crossing restriction. Even when ATC allows a descent at the pilot’s discretion, aircrews need to choose a waypoint and altitude for positioning convenient to start the approach. In either case, descending from a cruising altitude to a given waypoint or altitude requires both planning and precise flying.

ATC may ask the pilot to descend to and maintain a specific altitude. Generally, this clearance is for en route traffic separation purposes, and pilots need to respond to it promptly. Descend at the optimum rate for the aircraft being flown until 1,000 feet above the assigned altitude, then descend at a rate between 500 and 1,500 fpm to the assigned altitude. If at any time, other than when slowing to 250 KIAS at 10,000 feet MSL, the pilot cannot descend at a rate of at least 500 fpm, advise ATC.

The second type of clearance allows the pilot to descend “… at pilot’s discretion.” When ATC issues a clearance to descend at pilot’s discretion, pilots may begin the descent whenever they choose and at any rate of their choosing. Pilots are also authorized to level off, temporarily, at any intermediate altitude during the descent. However, once the aircraft leaves an altitude, it may not return to that altitude.

A descent clearance may also include a segment where the descent is at the pilots’ discretion—such as “cross the Joliet VOR at or above 12,000, descend and maintain 5,000.”This clearance authorizes pilots to descend from their current altitude whenever they choose, as long as they cross the Joliet VOR at or above 12,000 feet MSL. After that, they are expected to descend at a normal rate until they reach the assigned altitude of 5,000 feet MSL.

Clearances to descend at pilots’ discretion are not just an option for ATC. Pilots may also request this type of clearance so that they can operate more efficiently. For example, if a pilot was en route above an overcast layer, he or she might ask for a descent at his or her discretion to allow the aircraft to remain above the clouds for as long as possible. This might be particularly important if the atmosphere is conducive to icing and the aircraft’s icing protection is limited. The pilot’s request permits the aircraft to stay at its cruising altitude longer to conserve fuel or to avoid prolonged IFR flight in icing conditions. This type of descent can also help to minimize the time spent in turbulence by allowing pilots to level off at an altitude where the air is smoother.

Inappropriate descent planning and execution during arrivals has been a contributing factor to many fatal aircraft accidents. Since the beginning of commercial jet operations, more than 9,000 people have died worldwide because of controlled flight into terrain (CFIT). CFIT is described as an event in which a normally functioning aircraft is inadvertently flown into the ground, water, or an obstacle. Of all CFIT accidents, 7.2 percent occurred during the descent phase of flight.

The basic causes of CFIT accidents involve poor flight crew situational awareness, or SA. One definition of SA is an accurate perception by pilots of the factors and conditions currently affecting the safe operation of the aircraft and the crew. The causes of CFIT are the flight crews’ lack of vertical position awareness or their lack of horizontal position awareness in relation to the ground, water, or an obstacle. More than two-thirds of all CFIT accidents are the result of an altitude error or lack of vertical SA. CFIT accidents most often occur during reduced visibility associated with instrument meteorological conditions (IMC), darkness, or a combination of both.

The inability of controllers and pilots to properly communicate has been a factor in many CFIT accidents. Heavy workloads can lead to hurried communication and the use of abbreviated or non-standard phraseology. The importance of good communication during the arrival phase of flight was made evident in a report by an air traffic controller and the flight crew of an MD-80.

The controller reported that he was scanning his radarscope for traffic and noticed that the MD-80 was descending through 6,400 feet. He immediately instructed a climb to at least 6,500 feet. The pilot returned to 6,500 feet, but responded to ATC that he had been cleared to 5,000 feet. When he had read back 5,000 feet to the controller, he received no correction from the controller. After almost simultaneous ground proximity warning system (GPWS) and controller warnings, the pilot climbed and avoided the terrain. The recording of the radio transmissions confirmed that the aircraft was cleared to 7,000 feet and the pilot mistakenly read back 5,000 feet then attempted to descend to 5,000 feet. The pilot stated in the report: “I don’t know how much clearance from the mountains we had, but it certainly makes clear the importance of good communications between the controller and pilot.”

ATC is not always responsible for safe terrain clearance for the aircraft under its jurisdiction. Many times ATC issue en route clearances for pilots to proceed off airway direct to a point. Pilots who accept this type of clearance also are accepting the shared responsibility for maintaining safe terrain clearance. Know the height of the highest terrain and obstacles in the operating area and your position in relation to the surrounding high terrain.

The following are excerpts from CFIT accidents related to descending on arrival: “…delayed the initiation of the descent…”; “Aircraft prematurely descended too early…”; “…late getting down…”; “During a descent…incorrectly cleared down…”; “…aircraft prematurely let down…”; “…lost situational awareness…”; “Premature descent clearance…”; “Prematurely descended…”; “Premature descent clearance while on vector…”; “During initial descent…” [Figure 9]

Practicing good communication skills is not limited to just pilots and controllers. In its findings from a 1974 air carrier accident, the National Transportation Safety Board (NTSB) wrote, “…the extraneous conversation conducted by the flight crew during the descent was symptomatic of a lax atmosphere in the flight deck that continued throughout the approach.” The NTSB listed the probable cause as “… the flight crew’s lack of altitude awareness at critical points during the approach due to poor flight deck discipline in that the crew did not follow prescribed procedures.”

In 1981, the FAA issued 14 CFR Part 121, § 121.542 and Part 135, § 135.100, Flight Crewmember Duties, commonly referred to as “sterile flight deck rules.”The provisions in this rule can help pilots, operating under any regulations, to avoid altitude and course deviations during arrival. In part, it states: (a) No certificate holder should require, nor may any flight crewmember perform, any duties during a critical phase of flight except those duties required for the safe operation of the aircraft. Duties such as company required calls made for such purposes as ordering galley supplies and confirming passenger connections, announcements made to passengers promoting the air carrier or pointing out sights of interest, and filling out company payroll and related records are not required for the safe operation of the aircraft. (b) No flight crewmember may engage in, nor may any pilot in command permit, any activity during a critical phase of flight that could distract any flight crewmember from the performance of his or her duties or which could interfere in any way with the proper conduct of those duties. Activities such as eating meals, engaging in nonessential conversations within the flight deck and nonessential communications between the cabin and flight deck crews, and reading publications not related to the proper conduct of the flight are not required for the safe operation of the aircraft. (c) Critical phases of flight include all ground operations involving taxi, takeoff and landing, and all other flight operations conducted below 10,000 feet, except cruise flight.