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 Issue 533 June 2024 

Flight safety is, in large part, contingent upon an aircraft’s separation from
terrain and other aircraft. In today’s world of crowded skies, reduced vertical
separation, and automation during most phases of flight, Vertical Navigation
(VNAV) operations are paramount, and aircrew discipline is crucial.

VNAV systems are intricate, requiring complex interactions with other aircraft
automation and especially with aircrews. Operational modes can be confusing.
Misunderstanding mode details or nuance, incorrectly programming, or mismanaging
the VNAV can produce dangerous aircraft responses that are unexpected and
unwanted by pilots.

This month CALLBACK shares incidents revealing VNAV complexity, the importance
of VNAV discipline, and the short time it takes for problems to escalate and
safety to deteriorate when VNAV discipline is compromised. Have you ever asked
any of these questions of your VNAV?

Part 91 – Why Did It Do That?

This corporate Hawker 750 First Officer (FO) shares what might appear to be a
simple altitude deviation. The event and related factors both reveal some
confusion and provide insight into mitigating the problem.

■ While descending for the RNAV approach…we had a brief altitude excursion. I
was the Pilot Monitoring (PM) and second in command for the flight. We were
cleared for the RNAV via the VOR, [an assigned] fix, straight in, and told to
maintain 7,500 feet to the VOR. As I was looking down programming the Flight
Management System (FMS), I looked up and noticed that the Captain had descended
below our altitude assignment of 7,500 feet and went as low as 7,100 feet as I
recall. I called out the altitude immediately and asked the Captain why the VNAV
had been turned off. At that same time, Approach advised us of a low altitude
alert, and we were told to climb to 7,500 feet We climbed back up to 7,500 feet
and completed the approach with no other issues. At no time did ATC advise us of
a pilot deviation, and they never asked us to call a phone number.

When flying RNAV approaches, our company procedure is to make sure all crossing
restrictions are entered into the FMS and checked. If an altitude does not meet
the crossing restriction, we enter the crossing restriction altitude for each
assigned fix. Then we put the final approach fix altitude in the altitude
preselect [window] and use the VNAV button to descend and meet all crossing
restrictions, while monitoring the pink altitude in the top right-hand corner of
the PFD to make sure the airplane knows what altitude to descend to. This was an
RNAV approach to LNAV/VNAV minimums, so we would select Approach mode after the
airplane was within 30 degrees of the final approach course and established
inbound for the approach. We followed this procedure for this approach, however,
somehow the VNAV button got disengaged, which led to the airplane descending
below our crossing altitude of 7,500 feet at the VOR.

The problem was compounded when I diverted my attention to the FMS and was not
watching the Captain’s autopilot selections or monitoring our altitude. The VNAV
function was somehow disengaged. We descended toward the altitude selected in
the altitude preselect. [There were several] contributing factors. Weather [was]
in the area. Heavy rain shafts were all around the airport, and we were
discussing whether to continue or request vectors away. I was programming the
FMS and did not see the Captain descend below our altitude or press the VNAV
button to disengage it. We were in IMC, and the published missed [approach]
would have put us into the rain shafts, so we were discussing alternate missed
approach procedures and relayed the request to ATC. We…also had some confusion
about accepting the RNAV [approach] from the VOR [via] the assigned fix straight
in, or just from the assigned fix straight in.… We had been given an altitude of
7,500 feet to maintain, instead of 8,100 feet like on the chart. Our pilots and
other pilots are confused whether 7,500 feet is legal or…we would need to cross
at 8,100 feet like stated on the chart.



Part 135 – Where Is the Missing Waypoint?

This fractional Learjet Captain describes quickly escalating confusion and
difficulty in programming and executing an RNAV approach. A waypoint and its
crossing restriction appear to be missing in the lateral and VNAV automation.

■ A Learjet 60 descended below the MEA and intermediate fix crossing altitude on
the RNAV (GPS) [approach]. Subsequently a go-around was issued by ATC, and the
approach was flown again to a successful landing. We were originally issued a
hold on the way to the airport; all airplanes requesting the RNAV (GPS) approach
were issued holds. Another aircraft questioned how an airplane just landed and
was told that the only airplanes getting in are flying the localizer approach.
We then requested the localizer approach, and we set up and briefed the approach
as published. As we were being vectored to join the localizer, we were then
issued the RNAV (GPS) instead, a few miles from the VOR. After adjusting the FMS
to now fly the GPS approach, we crossed the VOR at 13,000 feet and started the
approach. The fixes in the FMS were as follows: VOR [Fix 1], and [Fix 3].
Missing from this approach in the FMS is the intermediate fix [Fix 2] that has a
crossing altitude of 12,900 feet. Not seeing this fix in the FMS and having
previously briefed a different approach with little time to set up and brief a
completely different approach, we proceeded down to the final approach fix
altitude of 12,200 feet at [Fix 3]. Upon leveling off, we were given a low
altitude alert from ATC stating the MEA in that sector is 13,000 feet. This was
the first indication on the [flight deck] that something wasn’t right.… We
immediately started a climb. We were then told that [Fix 2] has a crossing
restriction of 12,900 feet. After slight confusion on the [flight deck] for a
second of “what fix?” a go-around was initiated. Not even two seconds after
that, a go-around was issued by ATC. The go-around was flown as published and
the approach was then again initiated and flown to a successful landing. Further
exacerbating the situation was the FMS not recognizing the approach as an
approach, but simply waypoints…so the VNAV was not usable for the approach. To
avoid a similar situation, care should be given when accepting a new approach so
close to the initial approach fix when a different approach had been set up and
briefed. Further verification of any intermediate fixes and crossing altitudes,
not just the initial and final, would have helped had time permitted.



Part 121 – What’s It Doing Now?

A B767 Captain details the consequences of the crew’s VNAV interactions. The
Relief Pilot further reveals some sobering insight and explores the gravity of
the situation.

From the Captain’s report:

■ We began the approach, but updated weather indicated the airport was below
minimums, so we coordinated to hold. While in holding, [Company] advised that
another flight landed successfully, and with updated weather, we had the
visibility required to begin the approach. ATC amended our holding altitude from
5,000 feet to 7,000 feet, but we forgot to put our new cruise altitude in the
Flight Management Computer (FMC) like we did before attempting the first
approach. We received vectors to intercept the final approach course and
commenced the approach but did not recognize our lack of vertical guidance due
to not entering a new cruise altitude. The aircraft appeared to be flying the
approach in LNAV/VNAV passing the final approach point, but began a descent rate
approaching 1,500 fpm that wasn’t recognized. The Relief Pilot and Pilot Flying
(PF) began looking for approach lights as we approached minimums. They had the
approach lights in sight, and so we continued the approach, still descending
faster than planned. All of us were looking for the runway environment. At about
the same time the PF and Relief Pilot saw 4 reds on the PAPI, we received an
EGPWS terrain warning. I incorrectly called for a go-around instead of a CFIT
(Controlled Flight into Terrain) recovery, and during the maneuver, the pitch
attitude became excessive and we received a low airspeed caution as it decreased
to around 105 knots. We completed the maneuver, sorted through the distraction
of low fuel cautions due to our 10,000 pounds of fuel sloshing during the
go-around, and diverted to a nearby airport.

From the Relief Pilot’s report:

■ Following holding, the crew flew an RNAV [approach]. The crew made common
errors on the approach and ultimately descended inappropriately below the
minimum descent altitude using faulty visual cues.… The subsequent go-around
resulted in a “Caution Terrain” and then “Whoop-Whoop, PULL-UP.” … The descent
had inadvertently been continued during the go-around, which caused the GPWS
caution/warning. Then, the crew misapplied established procedures on
the…go-around, which resulted in excessively slow airspeed. I had to intervene
during both the RNAV approach and subsequent go-around to ensure safety. The
crew should have realized there was not a proper vertical path and either
modified [the] descent rate or discontinued the approach. Also, the crew should
have had the situational awareness to know that they were still several miles
from the approximate visual descent point and use that information when deciding
to proceed below the MDA. During the go-around, the FO became task saturated
with non-critical items (FMS, ATC communication, etc.) and failed to monitor the
flight path adequately and perform PM duties correctly. This greatly affected
the safety of flight during the go-around.



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ASRS Alerts Issued in April 2024 Subject of Alert No. of Alerts Aircraft or
Aircraft Equipment 4 Airport Facility or Procedure 8 ATC Equipment or Procedure
14 Other 1 TOTAL 27

April 2024 Report Intake Air Carrier/Air Taxi Pilots 6,770 Flight Attendants
1,802 General Aviation Pilots 1,595 Military/Other 904 Mechanics 354 Controllers
313 Dispatchers 305 TOTAL 12,043

NOTE TO READERS:
■ Indicates an ASRS report narrative
[  ]  Indicates clarification made by ASRS


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A Monthly Safety Newsletter from The Office of the NASA Aviation Safety
Reporting System
P.O. Box 189  |  Moffett Field, CA  |  94035-0189
https://asrs.arc.nasa.gov
Issue 533