GUEST COMMENTARY. In his Pelican's Perch columns #24 and #25, AVweb's John Deakin took the position that constant-angle non-precision approaches (CANPAs) may not be appropriate for use by other than glass-cockpit aircraft with sophisticated VNAV automation. Captain Erik Reed-Mohn of Scandanavian Airlines System (SAS) differs with that view, based on both his airline's experience flying such approaches in a wide range of transport jets from older round-dial DC-9s to the latest-generation glass-cockpit Boeings, as well as his participation in studies and working groups of the Flight Safety Foundation. The author illustrates his commentary with a number of actual approach procedures flown by SAS.
January 26, 2000
This article is written in response to Mr. John
Deakin's Pelican's Perch
columns #24 and #25 concerning
Constant-Angle Non-Precision Approaches (CANPA), in which Mr. Deakin takes the
position that such stabilized non-precision approaches may not be appropriate
for use by other than glass-cockpit aircraft with VNAV automation. I differ with
The concept of a stabilized constant-angle non-precision approach may be new
to most of us, but has in reality been used by some airlines for decades. My
first encounter with the concept came during my work on the Flight Safety
Foundation's (FSF) Controlled Flight Into Terrain (CFIT) Task Force, and its
successor, the Approach and Landing Accident Reduction (ALAR) Task Force. Both
the Flight Crew Training and Procedures Working Group of the CFIT Task Force and
the Operations and Training Working Group of the ALAR Task Force came up with
unequivocal support of the CANPA concept.
Having flown non-precision approaches the step-down way since my Air Force
days in the 70s, I must admit to an initial reluctance. But seeing the way the
statistics work out, with a concentration of wrecks along the extended
centerlines of runways, I did not need very much convincing that traditional
dive-and-drive (DaD) approaches were less than optimum. (See also Mr. Dick
Slatter's article "Chart Design Revision Could Enhance Safety of Non-Precision
Approach and Landing Operations", ICAO Journal May 1994.)
The clincher, as if that was needed, came with two reports published by the
- Airport Safety: A Study of Accidents and Available
Approach-and-Landing Aids. (Flight Safety Digest, March 1996.)
this report's conclusions were:
- "On a worldwide basis, there appears to have been a five-fold increase
in accident risk among commercial aircraft flying nonprecision approaches
compared with those flying precision approaches."
- "When stratified by ICAO region, the risk increase associated with
flying nonprecision approaches compared with those flying precision
approaches ranged from three-fold to almost eight-fold, depending on the
- An Analysis of Controlled-Flight-into-Terrain Accidents of Commercial
Operators, 1988 through 1994. (Flight Safety Digest, April-May 1996.) This
report showed that approximately 60% of CFIT accidents occurred within five
nautical miles (NM) from the threshold.
Mr. Don Bateman's statistics show that it is primarily navigation error in
the vertical plane which is responsible for CFIT accidents, since CFIT accidents
form a cluster along the runway centerline.
The point of trying to do something about these sad numbers was thus born out
of a desire to find a safer way of managing the vertical path during a
non-precision approach. The horizontal path seems to present fewer problems if
the numbers are to be believed, and I think they should be.
Not Just for Glass Cockpits
Mr. John Deakin seems to equate CANPA with VNAV or automated aircraft. While
they can certainly fly CANPAs too, this is a common misunderstanding that needs
As mentioned, some carriers in Europe, notably BA and KLM, have been doing it
this way for decades. No glass cockpits were available decades ago, and those
carriers (plus others who have seen the light) still operate steam-driven,
round-dial equipment and CANPA works fine there too. The fact that the first
U.S. operators to migrate to CANPA will probably have aircraft with VNAV
capability through the auto flight system does not change that fact.
To say that CANPA "is easier, but often it doesn't get the job done, it's not
the way the procedure was designed, and above all, it can be dangerous" to my
mind shows that Mr. Deakin may not be fully informed about developments in this
Maybe CANPA is easier. So what? Task overload had been shown to be a
significant factor in approach and landing accidents. Procedures that make our
lives easier should be welcomed. The FSF's Operations and Training Working Group
said it this way: "The data clearly show that task saturation and overload for
the pilot flying are significant contributors to Approach and Landing
Why wouldn't CANPA get the job done? As far as I can see, Mr. Deakin's
arguments against CANPA are just as applicable to the standard DaD concept. He
may be right that the classic manner "will quietly, legally, and safely make the
approach and landing while the glass cockpits go merrily to the alternate," but
I doubt it. Especially the safely bit.
Not the way the procedure was designed? Says who?
Both TERPS and PANS-OPS (the ICAO equivalent) have as a goal keeping the
pilots and their aircraft safely away from unwanted and premature contact with
the ground. They do not, however, in all details specify how a pilot should
operate his aircraft to stay within the design parameters of the approach
procedure design. To stay within the lateral boundaries, maximum speeds are in
some cases specified, but as far as vertical navigation goes, there are few
guidelines except: do not bust specified minimum altitudes.
What TERPS and PANS-OPS do not do is to specify that the pilot should
maximize their exposure to minimum terrain clearance the way Mr. Deakin
I would much rather do the wimpy gradual descent, with mental surplus
capacity to monitor the altitude restrictions which may apply, than wrench a 50-
or 250-ton airplane into a 1000 ft/min or worse, the 1500 ft/min descent
towards the MDA Mr. Deakin advocates. In my book, any descent rate in excess of
1000 ft/min close to the ground should be treated as dangerous. That is not to
say that it will not happen or that it cannot be handled safely, but it should
be treated with caution and carefully briefed and prepared for.
If this leads to more go-arounds and diversions, maybe that's just what the
The airline I work for, Scandinavian Airlines System (SAS), has its way of
doing this. We operate equipment ranging from round-dial DC-9s through the
latest generation B-737-600/700s and MD-80/90 and B-767 in between. All our
pilots use the same approach plates built on the same principles. They are based
on TERPS in the U.S. and PANS-OPS in most of the rest of the world. We do not as
yet fly GPS-based approaches.
We do not allow our pilots to design and fly auto flight guided VNAV
approaches. Putting the waypoints and altitude constraints into a flight
management system (FMS) is a very time-consuming task fraught with possibilities
for mistakes. In addition, the preset waypoints and altitude constraints put in
the FMS by the database producer do not permit easy corrections of minimum
altitudes for low temperatures. It is therefore a hard job to ensure the
vertical (obstacle clearance) integrity of a computer-based vertical path
displayed to the pilots.
FMS-equipped aircraft are, however, excellent at keeping the pilot honest in
the lateral dimension, provided the raw data are closely monitored.
We fly basically two types of non-precision approaches: those with DME
guidance and those without. NDB approaches are almost always without, and VOR or
LOC approaches may or may not have a DME associated with them.
In order to generate the reviled pseudo glide slope on an approach without
DME guidance, timing must be used. This is reflected in our guidance material to
our pilots. (See Figure 1.)
This is of course not enough. An operator must have clear and specific
guidance in place for its pilots to be able to interpret and fly complicated
procedures which all instrument approaches are. Accordingly our instructions
contain descriptions of how, for instance, the minima are presented. (See Figure
This shows an example of a time-vs.-altitude box on a non precision-approach
without DME guidance in this case, for the NDB approach with an MDA of 530
feet. OHT is the locator where the timing and final descent starts. Rate Of
Descent (ROD) vs. Ground Speed (GS) is given as guidance.
Here's a real life example from the U.S. (See Figure 3.)
This is our approach plate for Chicago KORD ILS 04R. What I want to
highlight here is the LLZ (LOC) approach without DME guidance. It is a TERPS
design (upper left corner) and has the altitude checks vs. timing. Timing and
final descent is performed from the Outer Marker (OM). MDA is 1290 feet.
In Figure 4, the principle applied to an approach with a DME vs. ALT table is
explained. Underlined altitudes are minimum altitudes and must not be penetrated
before the specified DME.
Figure 5: This is what it looks like in a real-life approach plate for
Charleston VOR-15. It is immediately obvious that the DME vs. ALT procedure is
simpler than the one with timing, but they are both eminently flyable.
It is furthermore not enough for pilots just to have a reasonable
understanding of how to interpret the approach plates when in flight. They must
have a thorough understanding of the principles applied in their design and the
inherent limitations in the presentations, which will invariably be present
either due to lack of space or simple neglect.
Figure 6 is used to illustrate such a point. There can be no doubts in
pilots' minds how to read this.
But even this is not enough. Somewhere in the operator's Standard Operating
Procedures (SOPs) must be a guide showing how to handle unintentional deviations
from for instance the ideal vertical path. Minimum altitudes must not be
penetrated until they no longer apply, and SOPs must handle unplanned deviation
events. Likewise it seems prudent to have a guide for how much deviation is
tolerated on advisory altitudes, with associated call-outs for the non-flying
pilot. "Slightly low, low, very low, slightly high, high, very high" and so
A Safer Way to Fly
Easy? Not exactly. Perfect? Not at all. But it is an attempt to get to grips
with the minimum altitude busts, unstabilized approaches and resulting premature
ground contact we all too often see in CFIT and approach and landing
I cannot guarantee that what I have presented here will be counted as great
progress in the fullness of time, but I can guarantee that we will have to
strive towards the objective of flying safer approaches. Technology will only
get us so far, although equipment like GPS with WAAS/LAAS and EGPWS hold great
Those of us who do not have this equipment today and will not have it until
earliest the day after tomorrow, must do with flying smarter in the
The CANPA is here for the taking.
[Editor's Note: Click here to read
John Deakin's response to