On the 4th of June 2023, a Cessna Citation 560 aircraft crashed in Montebello, Virginia, killing the pilot and three passengers. The NTSB released their report on the 14th of May, leading to a very Citation-heavy month this month on Fear of Landing!

The Cessna Citation 560, also known as the Citation V, is a light corporate jet for seven to eight passengers with twin engines and a range of about 1,700 nautical miles. As an upgrade to the Citation II, it has more powerful engines, improved aerodynamics and a greater range. It is pressurised, allowing for a maximum cruising altitude of 45,000 feet. Unlike larger business jets, the Citation 560 requires only one qualified pilot.

This particular Citation 560, registered in the United States as N611VG, was privately owned and based at Melbourne International Airport in Florida. The accident pilot was a “long-time friend of the family” and had managed their aircraft over the years.

That morning at about eleven, the pilot arrived at Melbourne Airport to prepare the aircraft for a general aviation flight. He departed around 11:00, flying to Elizabethton Municipal Airport in Tennessee to pick up three passengers, family members of the owners of the jet.

The pilot refueled and they departed Elizabethton at 13:13 for a flight to Long Island MacArthur Airport in Ronkonkoma, New York.

The pilot contacted Atlanta Air Route Traffic Control Centre shortly after take off to report climbing through 9,300 feet. The controller cleared the aircraft to 23,000 feet cruising altitude and then, after the pilot read back the clearance, handed the pilot off to another controller at Atlanta Center.

The pilot contacted the second controller and advised that the aircraft was maintaining 23,000 feet. The controller cleared the flight to 29,000 feet and then to 34,000 feet. The Citation 560 was climbing through 26,600 feet as the pilot read back the clearance to 34,000 feet.

Three minutes later, the controller called the pilot again, asking him to level off at 33,000 feet as there was conflicting traffic.

The pilot did not respond. The Citation 560 continued to climb to 34,000 feet and levelled off.  Fifteen minutes had elapsed since they had taken off from Elizabethton.

The controller repeatedly called the pilot but did not receive any response.

Alerted by air traffic control, someone from the FAA contacted one of the owners, asking if there was some alternate method of reaching the pilot on the aircraft. She said that there was no means of contacting the cockpit from the internet or a cell phone.

The Citation 560 continued towards its destination, making turns consistent with a route programmed into the flight management system at departure.

At 14:32, the aircraft arrived over Long Island MacArthur Airport, still at 34,000 feet, and continued to fly to the southwest. It was now heading directly towards Washington DC.

After attempts to contact the pilot continued to be ignored,  six United States Air Force F-16 fighter aircraft were scrambled from three different locations.

Two F-16s from Joint Base Andrews were the first to reach the silent aircraft. The Citation 560 looked to be in good shape, no sign of damage or icing. They could see someone in the left cockpit seat, slumped over to the right. They called on the radio and flew in front of the jet in intercept manoeuvres, lighting flares in an attempt to get his attention. It became clear that the person was incapacitated. They could not see if he was wearing a headset or an oxygen mask. The cabin window shades were open but there was no movement in the cabin area and they could not see whether there were any passengers on board.

The intercept pilots watched the aircraft pitch become unstable (began to “burble”) and then the Citation 560 began a slow roll to the right.

At 15:22, the aircraft began an uncontrolled spiral toward the ground, rotating clockwise while rapidly losing altitude. The F-16 entered a steep dive at 6,000 feet per minute to remain with the aircraft; however, the pilots lost sight of it as it descended into cloud at 7,000 feet.

It plunged into a densely wooded mountainous area near Montebello, in a fast and steep dive, tearing through the trees  before smashing into the ground and bursting into flames.

Searchers found one engine about 100 feet downhill from the main crater. The second engine had shattered into pieces. The violent impact had scattered the debris such that the investigators couldn’t examine the flight surfaces and controls. As the F-16 pilots had seen the Citation in flight with no visible damage, the timing of the final descent suggests that the autopilot exceeded its operational envelope or developed a system failure, leaving the aircraft entirely uncontrolled.

 The pilot had fallen out of contact at some point while climbing above 27,000 feet, which immediately suggested hypoxia.

Hypoxia, where the body becomes starved of oxygen, is a risk at high altitude without pressurisation.  The first symptom of oxygen starvation is that your judgement is impaired, accompanied by a general sense of well-being and euphoria, which means that the initial effects hide the hypoxia from the victim.  The “time of useful consciousness”, that is, the time in which you remain capable of making sensible decisions and correcting the problem, is limited.

The pilot’s medical history put him at some increased risk for hypoxia but there was no reason to believe that he might have become incapacitated for medical reasons or that he had taken anything that would have affected his ability to fly the aircraft.

The investigators focused on the aircraft’s cabin pressure and oxygen systems. They recovered a few components, but most of the critical pressurisation equipment was destroyed in the crash. Two passenger oxygen masks were found still attached to their box assemblies, which means that they had not dropped from the overhead compartments for the passengers to use. Another mask was found improperly capped with a dust cap instead of the regulation safety cap.

The Citation’s environmental control system used bleed air from the engines to pressurise and air condition the cabin. All the controls for this system were on the Pressurisation – Environmental Control panel in the cockpit. Here, the pilot could set their desired cabin altitude, temperature and airflow.

This panel displayed critical information including pressure differential, cabin altitude and rate of change. If the cabin pressure altitude climbed above 10,000 feet, a red CAB ALT 10,000 FEET light would illuminate, triggering the master warning system. This is a critical warning of a potential pressurisation issue, which prompts the pilot to use supplemental oxygen (the ultimate example of put your own mask on first before helping others).

Flight crew use sweep-on masks, a type of oxygen mask designed to quickly “sweep” onto your face in a one fluid motion, without needing to adjust straps or loops. These are placed below the cockpit side windows for quick access, as during a decompression event, the time of useful consciousness is very limited. For the cabin, continuous flow masks are stowed in overhead panels above each seat. These should drop automatically if the cabin altitude rises to between 12,900 and 14,100 feet, which signals a possible decompression event.

The flight crew manage the oxygen system from controls on the left console. An oxygen control valve switch can be used to manually drop the passenger oxygen masks if needed. It also allows the pilot to  control the flow of oxygen to flight crew only or to both flight crew and passengers (by default, it is set to both).

The oxygen is stored in a 76 cubic foot bottle in a compartment in the tail. If fully charged, it provides about an hour of oxygen for eight on board (two flight crew and six passengers). The oxygen level is shown on a pressure gauge on the right instrument panel.

 There was nothing in the wreckage to show how or why the pressurisation or supplemental oxygen systems might have failed. At least part of that mystery was solved when looking at the maintenance records.

These records painted a concerning picture. Five maintenance items were overdue, including an inspection of the co-pilot’s oxygen mask. A visual inspection a few weeks before the accident, on the 10th of May, listed 26 issues. These included problems like the emergency exit door seal sticking out, incorrect installation of the humidity regulator and a loose cabin temperature sensor. The owner chose not to have these issues addressed. Tangentially, this report also mentioned that the aircraft was equipped with a cockpit voice recorder.

The pilot later reported to the family that there was a problem with the newly installed avionics, which shut down after a few hours of flight. The Citation 560 was taken back to the same maintenance provider on the 1st of June as a result of the pilot’s report.

During another visual inspection on the 2nd of June, a mechanic noted that the aircraft still presented the same 26 discrepancies noted on the 10th of May, with one critical difference. The pilot-side oxygen mask was not installed. The mechanic also noted that the oxygen level was at its absolute minimum serviceable level.

This was two days before the accident. There is no indication that the oxygen system was topped up or that the pilot’s mask was put back before the flight.

The Citation’s flight manual was clear: before every flight, the pilot should check that the sweep-on oxygen masks were in the cockpit. However, as this wasn’t reported before the aircraft went back to maintenance, it seems that the pilot was not checking for his own mask as a part of the pre-flight check. The before starting engines checklist also included a specific step for the oxygen system: CHECK and STOW. This meant verifying that a green band was visible in the O2 supply line, that the quantity gauge showed 1600-1800 PSI and that the crew masks were connected to their outlets. For flights above FL 250 (25,000 feet), crew masks needed to be stowed on the quick-donning hook and set to 100%.

If the pilot had performed these checklist items before starting the engines, he would immediately have seen that his own mask was missing and that the oxygen quantity was low. At the levels reported by the mechanic two days before the flight, no oxygen would have been available for anyone on board and the passenger masks would not have deployed in an emergency.

The NTSB concluded their investigation with the following probable cause:

Pilot incapacitation due to loss of cabin pressure for undetermined reasons. Contributing to the accident was the pilot’s and owner/operator’s decision to operate the airplane without supplemental oxygen.

Personnel issues: Hypoxia/anoxia – Pilot

Personnel issues: Decision making/judgment – Pilot

Personnel issues: Decision making/judgment – Owner/builder

Aircraft: (general) – Not serviced/maintained

Simply put: the Citation 560 was not equipped to handle a decompression event. We don’t know why the pressure system failed and it wasn’t possible to tell from what was left of the plane after the crash. However, it clearly did fail. When the pilot failed to notice the cabin pressure warning and continued to climb, they were already doomed.