On the 3rd of July in 2014, a Boeing 777-300ER operated by Singapore Airlines was involved in a “loss of separation” incident after departing Houston. The NTSB chose not to investigate the occurrence formally, which was regarded as a pilot deviation from an ATC clearance. However, they reviewed the circumstances of the incident and passed the details to the Air Accident Investigation Board of Singapore, including an initial report by the Federal Aviation Administration. The AAIB of Singapore considered the occurrence to be significant and created their own report to highlight the safety lessons of the incident. This report can be downloaded as a PDF here.

The Boeing 777-300ER, registration 9V-SWH, was operated by Singapore Airlines for scheduled passenger flights.

That day, the Singapore Airlines crew prepared their Boeing 777 for their international flight from Houston, Texas to Moscow. They had arrived from Moscow two days previous. The flight crew consisted of two captains and a senior first officer. One of the captains took the role of Pilot Flying and the senior first officer took the role of Pilot Monitoring.

The flight was departing from Houston’s runway 15 left. As they prepared for the flight, they planned their route on the expectation that they would receive ATC clearance for the Standard Instrument Departure route (SID) known as the INDIE ONE RNAV departure.

A Standard Instrument Departure is a procedure used at many busy airports to simplify giving clearances to crew. A SID should offer the flight crew a straight-forward departure via a number of waypoints or fixes which is documented in the airline charts. The SID should be easy to understand and, if possible, limited to one page. ATC will issue a specific SID based on a combination of the destination, the first waypoint in the flight plan and the take-off runway in use.

In the United States, the name of a SID refers to some notable characteristic of the procedure, often a waypoint, along with a single digit which signifies the version of the procedure. There is a waypoint called INDIE near the airport and this particular SID routed the aircraft north-north-east away from the airport via this waypoint. It was the first revision of that route (ONE) and the flight-crew would follow the beacons using area navigation (RNAV). If you were to fly out of Houston today following the SID, you would be given INDIE FOUR as there have now been three revisions to the INDIE SID.

A SID must have a textual description. In the Jeppesen charts, which were the charts that the crew was using, there was a route description followed by a pictorial version of the SID showing tracks and distances.

The text read as follows:

If you are wondering about that waypoint called SUSHI, well, Houston has a number of interestingly named waypoints, which are only restricted in that they must be five letters, easy to pronounce and not similarly named to any other waypoints in the general area. Houston waypoints include SSLAM and DUUNK, as well as TQELA, WORUM, CRVZA, CARNE and QUESO. I have to admit, I’m not sure that CRVZA fits the bill of being easy to say if you can’t guess the inspiration!

Looking at INDIE ONE RNAV, the flight crew needs to travel via the following waypoints: RENNK, COLET, SUSHI and INDIE. At INDIE, they would be cleared to follow their filed flight plan navigation.

In any event, the instructions are clear: if you are departing from 15L, you climb heading 149° to 600 feet, and you should expect ATC to offer you radar vectors to the waypoint called RENNK. At RENNK you should turn north to 016° until COLET, then 025° to SUSHI, then 026° to INDIE. You must maintain 4,000 feet or the altitude assigned to you by ATC with the clearance. ATC should clear you for your cruising altitude around ten minutes after departure. On this day, the flight crew’s initial cruising altitude was Flight Level 310 (31,000 feet).

The captain, as pilot flying, conducted a departure briefing with the Pilot Monitoring. The second captain followed the briefing. Each pilot had an Electronic Flight Bag (EFB), a tablet holding flight management information, and each EFB included a copy of the Jeppesen charts.

This was the first time that any of the crew had flown the INDIE ONE RNAV departure. The captain read the first line of the “Routing” section and then scrolled to the picture at the right of the chart. He zoomed in to check the track and distances. He ended his briefing without zooming back out or scrolling back to the routing section.

The Mode Control Panel (MCP) controls the advanced autopilot systems and related systems. It has many different modes (hence the name) to allow the flight crew to select which parts of the flight should be controlled automatically. Depending on the mode and settings, the MCP will instruct the autopilot to hold a specific altitude or to change altitudes at a specific rate. The flight crew use it to establish altitudes, speeds and climb/descent profiles.

As the captain concluded his briefing, he stated that there was no altitude restrictions.

The flight crew expected that Air Traffic Control would give them some intermediate altitude to level off at before they were cleared to FL310. The captain said later that as he didn’t see any intermediate altitude on the SID chart, he thought that ATC would give him one later. If ATC did not give him an intermediate altitude, he planned to ask them. However, he never mentioned his plan to ask ATC to the Pilot Monitoring, who later might have reminded him to ask.

The Flight Deck Preparation – Before Start checklist calls for the assigned altitude or SID limit altitude to be set in the altitude window on the Mode Control Panel.

The captain set the altitude window on the Mode Control Panel to FL310 (31,000 feet), the initial cruising altitude, which he described as the “next best thing as there was no other altitude to set”.

The Pilot Monitoring received an ATC departure clearance to Moscow, “via INDIE ONE RNAV departure, TPAKK transition, climb via SID.”

As they departed on runway 15L, the Pilot Monitoring contacted ATC. “Singapore 61. Take up heading of 020 passing 2,500 feet.” This means that the aircraft was at that moment climbing through 2,500 feet. Generally, the call is accompanied by a statement of the altitude that the aircraft was climbing to, for example “passing 2,500 feet to FL310”. In fact, the Singapore Airlines Flight Crew Training Manual is clear that pilots should state both the passing altitude and the assigned altitude in their initial contact with a radar departure frequency. In this case, the Pilot Monitoring did not ask what altitude they were cleared to for the departure and he did not include the set altitude of FL310 in his call.

As they were climbing through 5,600 feet, an alert sounded in the cockpit: TRAFFIC, TRAFFIC!

A Traffic Collision Avoidance System (TCAS) is installed into all modern commercial aircraft. TCAS interrogates the transponders of all nearby aircraft, receiving their altitude and distance. It uses this information to alert the flight crew of nearby aircraft. In the first instance, it will alert with a traffic advisory (TA). The flight crew are not (yet) expected to perform avoidance manoeuvres in response to a TA. The purpose of the TA is to make the flight crew aware that there is the possibility of a conflict, which gives the flight crew a chance to locate the other aircraft visually and prepare for an evasive manoeuvre if needed.

The Traffic Alert and Collision Avoidance System had generated a traffic advisory (TA).

An air traffic controller contacted the flight crew at the same time and urgently instructed them to descend to 5,000 feet. A conflicting aircraft had been cleared to descend to 6,000 feet and the two aircraft no longer had the 1,000 feet of vertical separation required.

The captain disconnected the autopilot and manually initiated a descent.

If the TCAS determines that there is a real risk of collision and, if the other aircraft is also TCAS-equipped, the TCAS will send a coordination signal to the other aircraft in order to resolve the encounter and then broadcast a resolution advisory (RA) in the cockpit.

In this way, the evasive manoeuvres are coordinated: one TCAS system will select an “upward sense RA” (instructing the pilot to climb) and the coordination signal to the second TCAS means that the second system will select a “downward sense RA” (instructing the pilot to descend).

Generally, the RA per aircraft is chosen that the aircraft with the higher altitude is advised to climb and the aircraft at the lower altitude is advised to descend, thus immediately increasing the vertical separation between them. However, in rare instances, the best way to separate the aircraft includes the two aircraft crossing altitude. In this instance, the higher aircraft was already descending fast and the Singapore Airlines 777 had been climbing. So the TCAS resolution was for the Singapore Airlines 777 to continue to climb, although it meant that they would cross altitudes.

The RA in the cockpit reflected this so that the flight crew would be aware: CLIMB, CROSSING, CLIMB! CLIMB, CROSSING, CLIMB!

Under FAA rules, the flight crew are expected to follow the resolution advisory “unless doing so would jeopardize the safe operation of the flight or the flightcrew can ensure separation with the help of definitive visual acquisition of the aircraft causing the RA”. The worst case scenario is when one aircraft follows the advisory and the other aircraft disregards it and does the opposite, which means that the aircraft, already at risk of collision, are heading towards each other.

That’s what happened in this instance. The conflicting aircraft received the resolution advisory DESCEND, CROSSING, DESCEND! so it continued its descent, expecting the other aircraft to climb. The Singapore Airlines Captain, however, continued his descent as well.

They were lucky. A second resolution advisory came in, instructing the flight crew to level off. The aircraft was still descending. Eight seconds later, the TCAS sounded that the conflict had been avoided with the message Clear of conflict. The aircraft started to level off. The resolution advisory episode lasted 27 seconds and standard vertical separation of 1,000 feet was restored in less than ten seconds.

The flight crew reported to ATC that they were clear of conflict. The captain reconnected the autopilot at which point the aircraft resumed its ascent. ATC immediately contacted the crew to ask why they were climbing, as the flight’s last instruction had been to descend to 5,000 feet.

The crew had forgotten that the MCP altitude window was set to FL310. The first officer, not doing very well in his role as Pilot Monitoring, changed the setting to 5,000 feet. The aircraft descended again and then maintained the assigned altitude of 5,000 feet.

The second captain in the cockpit spoke up, asking if ATC had assigned an altitude when they gave the flight its departure clearance. The first officer remembered, correctly, that no specific altitude had been assigned. The controller had cleared them to “climb via SID”.

If a Standard Instrument Departure contains a published altitude restriction or top altitude, then ATC will clear a flight to “climb via SID”, which means to comply with the published procedure. This is new phraseology in the US but Singapore Airlines had informed their crews about the procedures and done training on Standard Instrument Departures as a part of their simulator sessions.

The second captain pulled up his copy of the Jeppesen chart. He read it aloud to the captain and the first officer: “MAINTAIN 4,000 FT or as assigned by ATC.” All three of them had missed the altitude restriction during the departure briefing.

The captain had scrolled and zoomed in to a specific part of the chart without ever taking the time to read the Routing section, which included the altitude restriction. The first officer should have been monitoring the briefing but he didn’t notice the altitude restriction either, nor did the second captain. This is a clear failure of crew resource management (CRM) in the cockpit.

The flight crew did notice that there was no intermediate altitude to level off before being cleared to the final cruising altitude. However, they never discussed this. No one commented when the captain set FL310 on the MCP altitude window. The pilot monitoring never asked the question when speaking to the controller and he didn’t specify the altitude they were climbing to when he made the initial call. If he’d followed standard operating procedure, the controller would have stopped the climb at 4,000 feet, keeping the aircraft out of the way of the descending traffic.

When the controller saw the conflict and instructed the crew to descend to 5,000 feet, no one in the cockpit set the new altitude in the MCP. None of the flight crew noticed that it was still set to FL310 and once they had dealt with the conflicting traffic, they all simply left the aircraft to begin another unauthorised climb. It wasn’t until ATC directly asked the flight crew why they were climbing that anyone in the cockpit paid any attention to the problem at all.

Finally, there’s the issue that the flight crew did not comply with the TCAS resolution advisory. In interviews after the incident, the flight crew all confirmed that they understood that resolution advisories must be complied with even if they are in conflict with ATC instructions. The captain said that he had experienced resolution advisories before but had never heard the CLIMB, CROSSING, CLIMB advisory. Both the first officer and the second captain said they’d never heard a resolution advisory in actual flight before. The lack of familiarity might be why the captain continued his descent and the other two did not argue. In response to this incident, Singapore Airlines has incorporated more demanding TCAS resolution advisory scenarios in their simulator recurrent training programme, including scenarios where there are conflicting instructions from ATC to the RA commands and specific scenarios where the Pilot Flying is not following standard operating procedures and the Pilot Monitoring is expected to challenge the Pilot Flying or even take over the controls. The airline also issued circulars to remind flight crews to be aware of altitude restrictions as indicated on the SID charts and to check with ATC if there is any doubt as to what the intermediate altitude should be.

The US Federal Aviation Authority has also responded to this incident. The RNAV SIDs, including the INDIE procedure (which is now currently revision four and had the altitude restriction raised to 16,000 feet) have been amended to always show the top altitude in a box at the top centre of the chart.

As a result of these actions by the airline operator and the aviation authority, the AAIB of Singapore ended their report with no safety recommendations. Everything was already in progress.

I’m impressed that they took the time to put together an incident report simply for the learning experience that it provides.