Pilot, safety expert, consultant, and aviation journalist Stuart “Kipp” Lau writes about flight safety and airmanship for AIN.

Contributor - Safety

Stuart “Kipp” Lau

Full-regime lateral and vertical navigation systems were first introduced in third-generation air transport aircraft during the 1980s. Designed to ease workload, increase situational awareness, and improve flight path guidance, LNAV and VNAV systems are loved by generations of pilots. However, in contrast, these same automated systems—particularly VNAV—can be deemed overly complex and confusing by pilots, often resulting in unexpected or undesirable outcomes.
Mastery of VNAV systems requires a deep understanding of its operation, programming, management, and—most of all—a high level of pilot discipline.
As highlighted in a recent NASA ASRS Callback newsletter, the most common flight crew errors today are no different than those mistakes made in the past. While there is nothing novel about these errors, they are exacerbated by an increased reliance, within the U.S. National Airspace System, on RNAV SIDs, STARs, and space-based RNAV (GPS) instrument approaches.
<h2>Mode Awareness</h2>
In the first Callback example, the crew of a corporate-operated Hawker 750 found themselves 400 feet too low while flying an RNAV instrument approach to LNAV/VNAV minimums. There were several contributing factors during this event, including heavy rain around the airport and the pilot monitoring (PM) being too focused (“heads down”) on programming the FMS rather than watching the pilot flying (PF), mode selections, and autopilot status.
As reported, the company’s procedure was to set the final approach fix (FAF) altitude in the altitude pre-select window and use VNAV to descend and cross above any intermediate stepdown fixes. Once established on the final approach course, the approach mode would be activated, which enabled the aircraft to descend from the FAF altitude to the appropriate minimum descent altitude.
According to the PM, “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.” The PM, nearly simultaneously, noted that the aircraft had descended to 7,100 feet and that the VNAV button had been “turned off.” At about the same time, ATC advised the crew of a low-altitude alert and the crew then climbed back to 7,500 feet.
<h2>Trust but Verify</h2>
Next, a charter Bombardier Learjet 60 descended 800 feet below an intermediate fix altitude (and the minimum en route altitude) after having difficulty programming and executing an RNAV (GPS) approach. Time compression and an omitted intermediate fix from the FMC database contributed to this event.
According to the captain,&nbsp;the crew initially briefed and planned on flying a localizer approach. The captain continued, saying, “As we were being vectored to join the localizer, we were then issued the RNAV (GPS) instead…After adjusting the FMS to now fly the GPS approach, we crossed the initial approach fix at 13,000 feet and started the approach.” The captain then identified an intermediate fix with a crossing altitude of 12,900 feet was missing from the FMS.
The captain continued, “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 FAF altitude of 12,200 feet.” Upon leveling off, ATC issued a low-altitude alert and stated that the MEA for that sector was 13,000 feet. According to the report, this was the first indication to the crew that something was wrong, and they initiated a go-around.
In retrospect, the captain said, “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 been helpful.”
<h2>Sage Advice</h2>
According to the Flight Safety Foundation (FSF) Approach-and-Landing Accident Reduction<a href="https://flightsafety.org/toolkits-resources/past-safety-initiatives/approach-and-landing-accident-reduction-alar/">Toolkit</a> (Briefing Note 1.2 – Automation), to use the full potential of automation and to maintain situational awareness, an understanding of the interface between the pilot (human) and automation (machine) is required. To better understand the system, the pilot must answer the following questions:
<ul>
<li>What did I tell the aircraft to do?</li>
<li>Is the aircraft doing what I told it to do?</li>
<li>What did I plan for the aircraft to do next?</li>
</ul>
Likewise, FSF suggests that automated flight control systems (AFS) must always be monitored by:
<ul>
<li>Cross-checking the status of the autopilot/flight director and autothrottle modes (armed and selected) on the flight mode annunciator;</li>
<li>Observing the result of any target entry (on the mode control panel) on the related data as displayed on the primary flight and navigation displays; and</li>
<li>Supervising the resulting autopilot/flight director guidance and autothrottle operation on the primary flight and navigation displays.</li>
</ul>
In general, FMS entries are considered “strategic” (the plan), whereas direct entries on the mode control panel are more “tactical” to accomplish short-term modifications to the desired flight path.
Pilots must be proficient at using all levels of automation and be able to select a lower level of automation when required (to include reverting to hand flying). Also, pilots must engage in an active monitoring role to identify and correct flight path or energy state deviations.
Finally, according to FSF, the safe and efficient use of the AFS and flight management system is based on the following three-step method.
<ul>
<li>Anticipate: Understand system operation and the results of any action, be aware of modes being armed or selected, and seek concurrence with the other flight crewmembers.</li>
<li>Execute: Perform the action on the AFS control panel or on the FMS control display unit (CDU); and,</li>
<li>Confirm: Cross-check armed modes, selected modes, and target entries on the FMA, primary flight display and navigation display, and FMS CDU.</li>
</ul>
The opinions expressed in this column are those of the author and not necessarily endorsed by AIN Media Group.

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