CFIT-Procedure Design Considerations Use of VNAV

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OCP-WG-WP 4.18
OBSTACLE CLEARANCE PANEL
WORKING GROUP AS A WHOLE MEETING
ST. PETERSBURG, RUSSIA
10-20 SEPTEMBER 1996
Agenda Item 4: PANS-OPS Implementation
CFIT-Procedure Design Considerations
Use of VNAV on Conventional
Non-Precision Approach Procedures
WORKING PAPER
Presented by
J.W. GREGORY
SUMMARY
This paper identifies safety concerns regarding the use of the aircraft modern technology
capability of VNAV while flying conventionally designed non-precision approaches.
BACKGROUND
As the computer technology of aircraft navigation systems became more and more sophisticated, aircraft
and avionics manufacturers attempt to exploit this computer capability in aircraft operations. One of the
most profound capabilities being exploited recently is the aircraft’s capability of navigating vertically on
an instrument approach without reference to an external electronic guidance signal such as an ILS
glideslope or MLS elevation signal. This mode of operation is called “VNAV”.
The vertical guidance is usually based on barometric altimetry augmented with information from a mix
of navigation sensors. Vertical command information may be retrieved from the aircraft’s aeronautical
information database or from the pilot’s input into the Flight Management System (FMS). Vertical
command information while conducting VNAV on a conventional non-precision approach is normally
retrieved entirely from the aircraft’s aeronautical database.
CFIT
Operators of these new technology aircraft have been using the VNAV features for determining the Topof-
Descent (TOD) in order to gain the most economical benefit of operating the aircraft in the descent and
approach for landing. In addition, however, pilots have been utilizing the VNAV capability of a large air
carrier type aircraft to establish a stabilized descent profile while conducting a non-precision approach.
Traditionally, aircraft had descended in steps to level at the Minimum Descent Altitude (MDA) during
the conduct of a non-precision approach. This “de-stabilized” method of flying an instrument approach
procedure is considered by many to be a major contributing factor in Controlled Flight Into Terrain
(CFIT) accidents.
Much has been written concerning CFIT while conducting an instrument approach procedure. In the
effort to reduce CFIT accidents during non-precision approaches, VNAV has been touted as the most
effective means to manage the vertical component of a non-precision approach procedure by avoiding the
necessity of levelling-off at the minimum flight altitude along each of the different segments of the
procedure. Some operators have extended the philosophy of VNAV to the point of describing it as an
“ILS look-alike.” Others have readily accepted the use of VNAV on non-precision approaches as simply
another precision approach!
DEVELOPMENT OF APPROACH PROCEDURES
All members of the OCP are acutely aware of the differences in developing non-precision and precision
approaches and the associated definitions:
Minimum descent altitude (MDA) or minimum descent height (MDH). A specified
altitude or height in a non-precision approach or circling approach below which descent
must not be made without the required visual reference. (PANS-OPS VOL II)
Decision altitude (DA) or decision height (DH). A specified altitude or height in the
precision approach at which a missed approach must be initiated if the required visual
reference to continue the approach has not been established. (PANS OPS VOL II)
Precision approach procedure. An instrument approach procedure utilizing azimuth and
glide path information provided by ILS or PAR. (PANS OPS VOL II) NOTE: It has been
recognized that other signal-in-space glide paths, such as MLS, is included in this definition.
It appears, however, that with the introduction of VNAV on non-precision approaches, the differences
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between the traditional non-precision approach and precision approach in the minds of some authorities
and operators have been clouded and mis-represented. One State’s air carrier authority has gone as far as
authorizing air carrier operations utilizing VNAV on non-precision approach to permit the pilot to use
the MDA as a DH. In other words, the pilot is permitted to fly VNAV vertical guidance information on a
non-precision approach and when the barometric altitude reads the MDA value, a missed approach is
initiated if the required visual reference is not established. This, of course, allows the aircraft to be below
the MDA while transiting to the missed approach on a procedure that was developed requiring no
descent below MDA unless the required visual reference is established. No acceptable or unacceptable
height loss parameters are identified or stated within this authorization. This situation raises the first
safety concern regarding VNAV.
VNAV SAFETY CONCERNS
Descents below MDA without the required visual reference
In all ICAO States’ regulations, descents below the traditional non-precision approach MDA is strictly
prohibited without the required visual reference. These regulations governing pilot actions regarding
MDA are consistent with and support the instrument procedure design criteria of non-precision approach
procedures.
Descents below MDA on instrument approach procedures that have not been assessed for obstacles for
IMC descents below the published MDA
One of the reasons that the required obstacle clearance for a precision approach (ILS) procedure is less
than that of a non-precision approach is that on a precision approach, the vertical guidance is provided
by a signal-in-space ground-based transmitter. This signal-in-space glide path is not affected by altimeter
errors associated with VNAV and is monitored and flight checked to ensure compliance with the
standards. The instrument procedure development criteria recognizes this and appropriately applies a
reducing obstacle clearance requirement in the final segment as compared to a non-precision approach
flat required obstacle clearance.
Non-precision approach minimum altitudes are designed without recognition of any vertical guidance,
VNAV or otherwise. A controlling obstacle within the final approach segment may be located anywhere
along the segment and any descent below MDA in IMC could place the aircraft in direct conflict with that
obstacle. The lowest required obstacle clearance (ROC) on a non-precision approach (LOC and VOR with
FAF) is 250 feet above the controlling obstacle within the final approach segment. This 250-foot ROC is
guaranteed only when the conditions under which the procedure is flown are ISA. Under normal
operating conditions, the ROC may be more and can be less than the 250 feet.
MDAs are recognized world-wide as a “do not descend below” altitude under IMC. This recognition is
consistent with all non-precision approach procedure design.
If descents below MDA in IMC are permitted, what is the minimum acceptable height loss below MDA?
FMS and NMS computed non-precision approach descent paths that ignore step-down fix altitude
restrictions and which, in some cases, cause premature descent below these step-down fix altitudes
resulting in loss of designed obstacle protection
All database instrument approach procedure minimum altitudes are encoded by a commercial vendor
without any certification process to ensure correctness or integrity of the data. Many errors have been
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found in current aircraft approach databases - some errors were minor and not safety related; others were
major and seriously affected the safety of flight. A recent case in point resulted in the following service
bulletin alert:
“Certain FMSs and NMSs compute an NPA glidepath, or pseudo-glideslope, as a straight line
between the Final Approach Fix (FAF) and the Missed Approach Point (MAP) altitudes as
coded in the Jeppesen approach database. During NPA operations, vertical deviation guidance
is provided to this path.
A problem has been noted in that some charted approaches include step-down fixes with
altitude restriction which penetrates this computed glidepath, and therefore the vertical
guidance provided by the FMS/NMS will cause premature descent below these step-down fix
altitudes, thus resulting in the loss of some obstacle protection. These step-down fixes are not
currently coded as part of the approach procedure database included in the FMS/NMS.
If the published approach procedure contains one or more step-down fixes between the Final
Approach Fix and the Missed Approach Point, DO NOT USE vertical guidance provided by
the Flight/Navigation Management Systems. Fly Non-Precision Approach procedures based on
the Pilot’s Altimeter and the applicable published approach procedures.”
VNAV operations conducted on non-precision approaches do not address the issue of database errors
and how to mitigate these hazards.
Flight crew confidence of flying VNAV and its association with an “ILS-look-alike”
Much has been written and said about computer generated vertical guidance being an “ILS-look-alike.”
In actual fact, VNAV and its presentation to the pilot in the cockpit does look like an ILS. While this kind
of presentation may be desirable, it introduces human factor issues that the cockpit display design
engineers may not have considered.
Witnessing a number of modern aircraft cockpit procedures while the crews were operating the VNAV
capabilities of the aircraft, it became evident in these cases that the crew were flying the VNAV with the
confidence and comfort of an ILS glidepath presentation. While this may be harmless and, indeed,
desirable at altitudes that are not associated with obstacle clearance, it is this same confidence that may
be detrimental to the safe operation of the aircraft on a non-precision approach flown with VNAV.
VNAV is not the same, in any way, shape or form, as an ILS (or MLS) glidepath yet cockpit procedures
do not address or recognize this difference. ILS (and MLS) glidepath is a signal-in-space; VNAV is not.
ILS procedures are obstacle protected for descents below published DH; flying VNAV on non-precision
approaches and using MDA as a DH are not protected. ILS precision approach procedures do not have
step-down fixes in the final segment; non-precision approaches do and VNAV does not address these
step-down fixes. The integrity of the ILS glidepath is checked and monitored; database derived VNAV
does not have any integrity.
It is apparent that flight crews do not appreciate the limitations of VNAV and do not address these
limitations when briefing for an approach using VNAV.
VNAV Database Minimum Flight Altitudes
VNAV information is retrieved from the aircraft’s database or, in some cases, from a VNAV command
input into the Flight Management System made by the flight crew. Extending the VNAV capability of
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the modern aircraft avionics to the approach phase of flight raises a number of database issues that have
not been resolved as yet.
All database minimum flight altitudes are encoded by database vendors based upon the State source
aeronautical information for a particular procedure. All instrument approach procedures are developed
to provided the MINIMUM flight altitude for each segment of a procedure. Database suppliers encoded
this minimum flight altitude into the aircraft’s database. This, of course, does not mean that the aircraft
must be flown at these minimum IFR altitudes. In fact, correction factors are assumed to be applied.
PANS-OPS VOL I states, in part, “3.5.4.5.2. It is assumed that the aircraft altimeter reading on crossing the fix
is correlated with the published altitude, allowing for altitude error and altimeter tolerances.” Table III-3-3 in
PANS-OPS VOL I details values to be added by the pilot to published altitudes in feet.
Since the database reflects only published altitudes, operating an aircraft in a VNAV capacity on a nonprecision
approach will mislead the pilot unless the pilot takes some corrective action to add values to
the database altitudes. Adding a correction value to published altitudes in a conventional operation of
aircraft, i.e., without VNAV, is a fairly easy matter - not so when dealing with VNAV and databases.
Remote Altimeter Sources
Instrument approach procedures may be developed based upon local and/or remote altimeter sources.
A database minimum flight altitude problem occurs when the procedure is based upon a part-time local
altimeter and a remote altimeter source at other times. Database suppliers will encode only those
altitudes that are published on the instrument procedure chart, therefore if the procedure is to be flown
based on a remote altimeter source, the procedure database altitudes presented to the pilot are in error
and no message is transmitted to the pilot that will alert him/her of the database error.
Cold Temperature Corrections
Minimum flight altitudes within an aircraft database are accurate only under ISA conditions. PANS-OPS
VOL I & II clearly state the corrections needed to the altimeter under non-ISA conditions. These
corrections are easily applied to the conventional, non-VNAV, aircraft by simply adding the correction
value to the published altitude and flying the result on the barometric altimeter.
Adding a correction factor to the aircraft VNAV database is not that easily accomplished or, in fact,
desirable. The computer calculation of VNAV is based upon the altitudes encoded within the database.
Therefore, the VNAV presentation to the pilot will be incorrect in all cases should a correction factor be
needed on the procedure. Having the crew manipulate the aircraft database within the terminal area at a
time when pilot workload is already high is undesirable.
A method of accounting for correction factors to the aircraft minimum flight altitude database must be
developed.
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RECOMMENDATIONS
In the spirit of reducing CFIT related accidents, the conduct of VNAV operations on conventional nonprecision
approaches must be introduced and conducted with care and full knowledge of the capabilities
and limitations of the aircraft VNAV system. Furthermore, flight crews must respect all minimum flight
altitudes associated with the non-precision approach, no matter what vertical guidance information may
or may not be present. It becomes quite obvious that aircraft technology has surpassed the concepts
under which most non-precision approach procedures have been developed.
I solicit the support of the Working Group in addressing CFIT and instrument procedure design to put
forward to the ANC the following recommendations:
1. In order to comply with the intent and spirit of aviation regulations and non-precision approach
procedure design, non-precision approach VNAV operations must be conducted in such a
manner so as not to violate the minimum descent altitude unless the required visual reference
has been established. This will necessitate adding a factor to the MDA that equals the height loss
during the conduct of a missed approach procedure. This will permit the aircraft to execute a
stabilized approach, and under all weather conditions where the ceiling is above the MDA, no
penalty is placed on the operation. There is a trade-off between flying a stabilized approach and
accepting a missed approach under weather conditions that equal the MDA. This approach to
VNAV operations is consistent with State regulations, the non-precision approach procedure
design philosophy and international application of MDA.
2. Flight crew cockpit VNAV approach procedures must include, in the approach briefing,
limitations and cautions associated with flying VNAV non-precision approaches. Limitations
and cautions should include but not be limited to:
a. effects of temperature corrections;
b. remote altimeter setting application, if required;
c. step-down fix altitude restrictions;
d. height loss factor and application to MDA; and
e. database altitude confirmation and its correctness related to the
published instrument approach chart.
3. Request aircraft and avionics manufacturers to provide data on the design of VNAV in order to
facilitate development of instrument procedure criteria that will accommodate VNAV capability.
This VNAV data should include the accuracy, integrity and continuity that is common to other
navigation aids and the associated instrument procedure.
4. OCP to develop obstacle clearance criteria specifically for VNAV approaches.
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