Articles, Training

What Goes up Must Come Down

Posted on Apr 4, 2020 by Thomas Close

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Mention navigation, and without conscious reflection, most pilots think about the lateral—tracking between points A and B.  But there’s the other dimension to consider, too—navigating vertically, between points U and D (Up and Down).

Going Up

Climb vertical navigation has three principal areas for consideration—Obstacle Departure procedures, SIDs (Standard Instrument Departures) and missed approach procedures.

Obstacle Departure procedures are textually—not graphically—described.  They are a bridge from the runway environment to the en route airways designed to miss obstacles in the immediate vicinity of that aerodrome’s individual runway.  While they’re essential for initial climb at aerodromes without towers or departure TRACONs, at many Class D aerodromes ATC nonetheless will begin an instrument clearance with “via the XYZ Departure Procedure for runway AB.” Read these minutely-detailed ODPs very carefully and generate a mental 3-D situational awareness understanding of what and where each step is required, and preset all heading bugs, track courses, altitudes and communication and/or navigation frequencies.   Many ODPs specify a required minimum climb gradient in feet per mile.  Precise translation of feet per mile into the feet per minute we use is ground speed specific (miles per minute times the VDP climb gradient) but given Cirrus initial climb groundspeeds of roughly 120 knots, doubling the ODP climb gradient yields approximately the required climb rate in feet per minute.

SIDs are graphic and textual depictions of a regional departure routing with altitude and/or airspeed constrained waypoints.  A given named SID may apply to multiple aerodromes in that region, but SIDs are specific to an individual runway at the named aerodrome from which the aircraft is departing.  Often a particular SID only exists for certain specific runways, so the runway must be included in selecting the SID from the navigational database. ATC will not verbally reiterate the vertical or airspeed constraints, but the pilot is nonetheless meant to comply with all of them.  Even though our Cirrus flight management system nav databases include these SID constraints, none of the existing auto flight equipment is capable of using VNAV as their climb pitch mode.

Missed approaches have both lateral components and vertical.  The first order of business is to stop descending or being level, and start CLIMBING.  Putting altitude between the plane and the ground or its obstacles is job one.  Naturally this means the simultaneous application of full power and coordinating right rudder pressure, then at safe speeds, with positive climb rates, retracting flaps on schedule, one notch at a time.  The lateral guidance of turns to headings, and courses to track are essential once past the missed approach point, but the climb can and should begin as soon as it is clear a landing will not be made.

Coming Down

It is a truism that it is difficult to pull off a fine landing out of an unstable, hurried, catch-up approach.  It is difficult to accomplish a stable, on the numbers approach from a poorly planned and executed descent from cruise.  Effective descent from cruise doesn’t just happen at the whim of the avionics or ATC, but results from the situational awareness and proactive planning and execution of the Pilot in Command.

Effective descent management involves planning a top of descent point, then validating the descent rate while underway.

Calculate the altitude to lose from cruise altitude to some altitude-defined aerodrome maneuvering point—an Initial Approach Fix, the Final Approach Fix, the runway elevation for a straight-in VMC landing, or the downwind pattern altitude—whatever applies.

Divide that altitude by a rate of descent appropriate to the region, its terrain, expected meteorological conditions, occupant comfort, or other factors.  A common strategy in mountainous areas or over areas with poor engine-failure landing sites might be at 1,000 FPM.  Over flatter terrain, perhaps 500 FPM is a comfortable rate.  Divide the altitude to lose by that rate to get the minutes needed for descent.  Multiply this by the cruise groundspeed in nautical miles per minute (groundspeed in knots divided by 60) to obtain the distance to the fix at which to start down. 

For instance, if in cruise at 180 knots groundspeed at 10,000’ MSL, and planning a 1,000 FPM descent to an IAF with a 3,000’ MSL crossing altitude, descent should be begun not less than 21 nm from the fix.  7,000 feet to lose at 1,000 FPM = :07 to descend.   180 knots groundspeed ÷ 60 = 3 nm/:01.  3 nm × :07 = 21 nm. 

Is this necessary if we have VNAV in our avionics package?  The PIC mindset is proactive as to all things in aviation, including descent point.  The avionics should always be used as a tool, not as guide, with the PIC having the situational awareness of where to start down, and crosschecking that with the avionics TOD.  VNAV in Perspective/GFC 700 and Avidyne R9/DFC 100 are outgrowths of jet “just in time” descent planning.  They calculate the top of descent point, and adjust the descent rate in real time for groundspeed variation on descent.  What is called VNAV with GNS or GTN hardware is merely a calculation of the top of descent point, and neither the STEC or DFC90 autopilots have VNAV pitch modes.

Perspective or R9 VNAV can be an effective descent tool for altitude-constrained flight plan waypoints, but it has three inherent limitations.  It will calculate and accomplish the descent even if there is no ATC authorization to do so.  For descent from cruise, not using any prescribed procedure such as a STAR, it could well take the aircraft into terrain or obstacles, or below MEAs/MOCAs.   And if used for Dive & Drive non-precision approach step-downs, it can never descend to the last (MDA) altitude, as this is invariably a contingent altitude.

STARs, or Standard Terminal Arrivals are graphic and textual routings for descent in busier metropolitan airspace, often serving multiple aerodromes in a region.  STARs are designed to direct the flow of traffic by pathway and altitude for ATC traffic separation, and to codify the pilots’ planning and operation.  STARs commonly have named waypoints with altitude and/or IAS constraints, and thus yield to descent planning as described above, or the use of VNAV as an auto flight descent pitch mode.  A “Descend via XYZ STAR” clearance intends for the PIC to calculate and initiate the descent to accommodate the charted waypoints’ altitude and airspeed constraints without ATC verbiage indicating this is effectively a descent at pilot’s discretion.  It is still appropriate to advise ATC of “leaving ABC (cruise altitude) on the XYZ arrival.”  STARs typically end with a codified vector heading at the last charted waypoint.

Whatever descent protocol is used, it is incumbent on the pilot to keep a running validation that the descent rate is likely to produce the desired level-off at the appropriate altitude-defined fix.  For this, there is a long-established professional pilot tool, called “Three for One,” but more accurately called “Three Hundred Feet per Nautical Mile,” which is based upon the angle of a classic ILS glide slope.

A 3.00-degree glide path descends at 318 feet per nautical mile, which can be rounded off with little error to 300’/nm.  Move the decimal point two to the left to make easy single-digit math, and you have a simple descent rate HOWGOZIT tool. 

For instance, while descending 20 nm away from an IAF with a 3,000’ MSL crossing altitude, the aircraft should be descending through 9,000’ MSL.  3 x 20 = 60, then add back the two zeros removed for the single digit math = 6,000’ + 3,000’ for the IAF = 9,000’ MSL.  Are you on a 5-mile straight-in VFR final to a runway?  You should be descending through 1,500’ AGL. 

Approach Types and Hardware Considerations

For approaches, one needs to decide if the procedure will be one with Vectors to Final (with it’s FAF crossing altitude), or will incorporate a transition (with its IAF crossing altitude).  If ATC does not have radar coverage at that location, it will have to be a transition appropriate to the runway and the arrival quadrant.  If this is a radar environment, either approach type is possible, although ATC will lean toward vectors, as this facilitates their maintaining aircraft separation.

Next, will the procedure provide a glide path to follow?  ILS approaches mostly include a glide slope.  LOC-only, Back Course, SDF, LDA, and VOR approaches do not.

For RNAV (GPS) approaches, non-WAAS/SBAS hardware does not have glide path capability, and will only annunciate APR in the avionics when the approach is active. For these, all GPS approaches are LNAV procedures, best flown, where possible, via a Constant Descent Final Approach utilizing a FPM descent rate appropriate to the groundspeed. Jeppesen charts include a rate vs. groundspeed table, not so NACO charts.  The descent rate can be approximated by multiplying the groundspeed in knots by 5, e.g. at 100 knots groundspeed, the rate would be approximately 500 FPM.  Dive & Drive can be used for these procedures, but carries considerable risk of vertical and airspeed destabilization.  Getting behind can lead to altitude busts, as well as stall/spin mishaps.

For GPS procedures to have a glide path, the aircraft must have functional WAAS equipment. WAAS hardware is not to be confused with a WAAS approach, although to gain the full benefit of a WAAS approach, WAAS hardware is required.   If an approach has the runway name in the approach title, e.g., RNAV (GPS) Rwy 26R, it is a procedure with Straight-In minima to that runway.  (There will also be Circle-to-Land minima for wind or other circumstances that require circling in VMC below the ceiling to land on a runway different than that of the S-I approach.)  The S-I approach allows the WAAS flight management system to create what is for all the world a carpenter’s snap line extending backwards and up from the runway touchdown zone to the FAF crossing altitude via what is known as the Vertical Descent Angle (VDA) in feet/nm.  Naturally, we fly it the other way around, but that is how the FMS does its trigonometric magic.  The VDA is part and parcel with the navigational database, allowing the system to generate a flyable glide path.

With SBAS equipment, with one exception, if the GPS procedure has S-I minima, there will be a glide path.  The annunciations for these procedures, in generally descending order of desirability are LPV, L/VNAV and LNAV+V.

If the S-I GPS approach has the WAAS term in the chart-briefing strip, the vast majority of times it will use a glide path to descend to LPV (Localizer Precision with Vertical guidance) minima, the GPS equivalent of a Category I ILS, and flown to a DA.   The one exception to GPS S-I IAPs having a glide path is also a WAAS-titled procedure where the most favorable minimum tabulation is labeled “LP” (Localizer Performance).   LP procedures are the GPS equivalent of the LOC only approach, and are flown to an MDA (Minimum Descent Altitude).   Jeppesen charts show the WAAS term in the briefing strip, and also in the plan view of the procedure.  NACO charts show WAAS in the briefing strip, but lack the plan view WAAS titling.

Another GPS S-I approach is one without the WAAS term in the briefing strip, and in which LNAV/VNAV (Lateral Navigation/Vertical Navigation) is the most favorable minimum shown.  While WAAS procedures will show LNAV/VNAV as a possible downgrade minimum, a true LNAV/VNAV, with that as the most favorable minimum is a very uncommon procedure, and will never show WAAS in the briefing strip.  LNAV/VNAV procedures annunciate an abbreviated L/VNAV in the avionics, and are flown to a DA.

A ubiquitous non-WAAS titled GPS S-I procedure is actually an LNAV approach, but being straight in to the specified runway, the flight management system (navigator) is capable of generating a glide path, and this will annunciate LNAV+V in the avionics display (Lateral Navigation with [Advisory] Vertical guidance).  There are no chart minima tabulations that indicate LNAV+V.  The chart for this type of approach will list LNAV as the minimum tabulation.  These are Straight-In GPS procedures that are not WAAS, and not LNAV/VNAV, and are flown to an MDA. There are no assurances that the LNAV+V glide path will be at or above any altitude-constrained waypoints between the FAF and the MAP (Missed Approach Point), although this has exceptions if using Jeppesen charts with the [bracketed] VDA shown in the side view and with the FPM/GS table.

All three of these glide path RNAV procedures generate the lateral and vertical deviation indications in the same way, the difference is the accuracy with which the charting entity has encoded the nav data, and the degree to which the procedure has been designed, surveyed, and flight tested as to vertical obstruction considerations.

Any IAP that does not include a specific runway in the approach title will be a procedure to the airport only, and will have only Circle-to-Land minima, will lack glide path capability, and are flown to an MDA.  WAAS and non-WAAS hardware treat these approaches in the same way.  The approach title to these will have –A, -B, -C suffixes, with letters from the beginning of the alphabet used in the title, and the –A meaning this is the chronologically first such Circle-to-Land procedure designed for the aerodrome, the –B the second, and so on. These only Circle-to-Land procedures do not preclude legally or safely making a straight-in landing if the ceiling and visibility are such that the aircraft can break out in a position to make a stabilized straight-in landing.

Certain Straight-In procedures to a named runway will have “end of the alphabet” suffixes in their title, commonly -X, -Y, -Z, e.g., RNAV (GPS) Y Rwy 26.  These S-I approaches are not to be confused with the “beginning of the alphabet” procedures to an airport (and which have only Circle-to-Land minima).  These –X, -Y, -Z suffixes are used to differentiate similar approaches to the same runway, and generally the later alphabet letters equate to more favorable minima than the earlier.  The -Z is commonly a WAAS procedure, which would annunciate LPV when active.

IAPs with RNP in the chart title (Required Navigational Performance) are Authorization Required approaches, and are unavailable to Cirrus aircraft.

Approach Procedural Methods

For all IAPs, plan descents so as to configure the aircraft on altitude with power appropriate to desired speed and intended flap configuration (usually ½ flaps) prior to reaching IAF for transition, or FAF for VTF. 

For approximately 100 KIAS: NA SR22, power is generally near 30% or 15” MP. TN = 50% or 15” MP.  SR22T = 40% or 15” MP.  For an SR20 = 50% or 22” MP.   For every level-off, promptly set this ballpark power setting and crosscheck airspeed, while coordinating the rudder.

To follow an approximately 3 degree glide path with the same flap configuration above, as descent is initiated, promptly reduce power to—NA SR22 15% or 12” MP, TN: 25% or 12” MP, T: 30% or 12.5” MP, SR20: 25% or 12” MP.  This will produce 500-600 FPM down.  For greater descent rates, power will need to be lower.

Power-up at level-off, and power-down at descent must be accomplished with a Pavlovian promptness and consistency, while simultaneously coordinating the rudder.

If descending a glide path, the power down and pitch over should be at glide path capture, so that the aircraft captures glide path and pitches over simultaneously. 

If descending Dive & Drive, the pitch over and power down can be initiated at 0.2 nm before the fix—with this small lead, by the time the aircraft actually leaves the altitude it is just passing the waypoint at 0.0 nm.   For Dive & Drive, plan 1000 FPM descent to intermediate waypoints and MDA, so as to not get caught high if autopilot begins shallowing out prior to ALT Preset, and allow a modest level period to look for the runway environment after breakout.

FAR 91.175 and the Visual Descent Point

FAR 91.175 c is the regulatory statute covering descent below DA or MDA.  It specifies the runway environment that must be in sight, and that the descent to landing must be at a normal rate and using normal maneuvers, which is FAA-speak for a stabilized descent from the DA or MDA after visually acquiring the runway.

Descent from DA or MDA does not assure obstacle clearance, which is why FAR 91.175 c requires visual acquisition of the runway environment—not the airport, but the runway, as defined in the regulation.

If flying a glide path, upon breakout, continue to follow the glide path to the touchdown zone.  This zone is depicted on runways with all-weather markings by two large rectangles either side of centerline, 1000 feet upwind of the threshold.  Resist the urge to duck under.  Do not destabilize the approach below DA or MDA.

However one arrives at the MDA on a non-precision straight-in IAP, the descent from the MDA must be stabilized as per FAR 91.175 c.  If flown as a constant descent final approach, it is stabilized into the MDA, and continues stabilized out of the MDA to touchdown.

The VDP (Visual Descent Point) is described in the AIM as a defined point on final for a non-precision straight-in approach from which a normal descent from MDA to the runway touchdown point may be commenced provided the runway environment of FAR 91.175 c is in sight.

A better way of rewording this VDP definition is that it is the point at which the descent from MDA must be commenced to assure a stabilized descent to a landing in the touchdown zone.  Not Before, and Not After the VDP.  Before carries obstacle collision risk, and after carries the risk of a non-stabilized descent at an excessive rate.

Both of these descriptions miss an important aspect of the VDP—it is that distance in nautical miles from the runway threshold at which a stabilized constant vertical descent angle from the FAF to the touchdown zone passes through the MDA.  Operationally this means that the VDP distance is the minimum visibility needed to complete a stabilized FAR 91.175 c descent from the MDA to a landing in the touchdown zone. 

The VDP is depicted on the approach chart with a large “V” and is discerned on the PFD.  Remember that the VDP distance is nautical miles, and the visibility conveyed on the ATIS or by the tower is in statute miles, which is 15% greater.  This VDP distance x 1.15 is what one needs as the minimum visibility to be heard on the broadcast to assure a stabilized descent from MDA and landing in the TDZ. 

And here is a GOTCHA—this distance is always higher than the minimum visibility published on the chart, often 2 to 3 times greater than the chart visibility minimum.   For these non-precision S-I IAPs, the reported weather must be a ceiling at/above the MDA and visibility at/greater than the VDP x 1.15 if a stabilized straight-in landing is to be accomplished.  If the ceiling is okay but the visibility is < the VDP in statute miles, that approach will have to be briefed and flown as a circle to land, with those chart minima.

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