Plane collides with Blackhawk helicopter Washington DC

[Zaphod58]

All the major components of both aircraft have been recovered and moved to a secure hangar, where the NTSB has begun going over them. 

[Zaphod58]

The NTSB has confirmed that there was a 100 foot discrepancy between the two altimeters on the UH-60. That's going to be the primary cause of them being too high on the route, and one of the links in the accident chain.

[KKLoco]

The NTSB has confirmed that there was a 100 foot discrepancy between the two altimeters on the UH-60. That's going to be the primary cause of them being too high on the route, and one of the links in the accident chain.

That's quite the discrepancy. How often are they recalibrated?

[Zaphod58]

That's quite the discrepancy. How often are they recalibrated?

They're fairly bulletproof, but they have to be set in flight based on the barometric pressure at the local airport. The most likely cause is that one of the pilots slightly misset the pressure on the altimeter on their side. That would result in a different altitude on that side.

[Zaphod58]

More in depth briefing from the NTSB:

They believe that the UH-60 crew was on NVGs, and there was no discussion of removing them on the CSMU, which would have been required before removing them. It was a combined annual evaluation and NVG training mission. The instructor was the Pilot Monitoring, which included all radio calls made by the crew, and the pilot being evaluated was the Pilot Flying.  

At 1.1 nautical miles west of Key Bridge, the PF indicated they were at 300 feet, while the PM indicated 400 feet. Neither pilot mentioned the altitude discrepancy.  As they approached Key Bridge the instructor indicated they were at 300 feet, descending to 200 feet. Passing over Memorial Bridge, the instructor said they were at 300 feet, and needed to descend. The PF indicated they would descend to 200 feet. 

At 8:46:01 a transmission was heard on the CRJ CVR telling the Blackhawk crew that traffic was a CRJ circling to runway 33, however the audio on the CSMU of the Blackhawk didn't include the word "circling". 

At 8:47:39 a radio transmission was audible on both CVRs asking if the Blackhawk had the CRJ in sight, with an audible "Conflict Alert" in the background.

At 8:47:40 the CRJ crew received an automated traffic alert from TCAS. There was no RA due to altitude.

At 8:47:42 ATC called the UH-60 and told them to pass behind the CRJ, although the CSMU didn't record the portion stating "pass behind the". The initial part of the transmission was stepped on by a 0.8 second mike key by the Blackhawk crew as they were radioing ATC. 

At 8:47:44 the Blackhawk crew indicated traffic in sight and requested visual separation which was granted. The instructor told the PF that he believed that ATC was asking them to move left towards the East bank of the Potomac. 

One second before impact the CRJ pitched up, reaching 9 degrees nose up with elevators near maximum nose up travel. The last recorded radio altitude was 313 feet two seconds prior, while at 9 degrees nose up and 11 degrees left roll. The radio altitude of the UH-60 was 278 feet, and had been steady for 5 seconds. Radio altimeter readings aren't the same as what's shown on the barometric altimeter however, so it's not clear what the pilots were seeing in the cockpit at this point.

[putnam6]

More in depth briefing from the NTSB:

They believe that the UH-60 crew was on NVGs, and there was no discussion of removing them on the CSMU, which would have been required before removing them. It was a combined annual evaluation and NVG training mission. The instructor was the Pilot Monitoring, which included all radio calls made by the crew, and the pilot being evaluated was the Pilot Flying.  

At 1.1 nautical miles west of Key Bridge, the PF indicated they were at 300 feet, while the PM indicated 400 feet. Neither pilot mentioned the altitude discrepancy.  As they approached Key Bridge the instructor indicated they were at 300 feet, descending to 200 feet. Passing over Memorial Bridge, the instructor said they were at 300 feet, and needed to descend. The PF indicated they would descend to 200 feet. 

At 8:46:01 a transmission was heard on the CRJ CVR telling the Blackhawk crew that traffic was a CRJ circling to runway 33, however the audio on the CSMU of the Blackhawk didn't include the word "circling". 

At 8:47:39 a radio transmission was audible on both CVRs asking if the Blackhawk had the CRJ in sight, with an audible "Conflict Alert" in the background.

At 8:47:40 the CRJ crew received an automated traffic alert from TCAS. There was no RA due to altitude.

At 8:47:42 ATC called the UH-60 and told them to pass behind the CRJ, although the CSMU didn't record the portion stating "pass behind the". The initial part of the transmission was stepped on by a 0.8 second mike key by the Blackhawk crew as they were radioing ATC. 

At 8:47:44 the Blackhawk crew indicated traffic in sight and requested visual separation which was granted. The instructor told the PF that he believed that ATC was asking them to move left towards the East bank of the Potomac. 

One second before impact the CRJ pitched up, reaching 9 degrees nose up with elevators near maximum nose up travel. The last recorded radio altitude was 313 feet two seconds prior, while at 9 degrees nose up and 11 degrees left roll. The radio altitude of the UH-60 was 278 feet, and had been steady for 5 seconds. Radio altimeter readings aren't the same as what's shown on the barometric altimeter however, so it's not clear what the pilots were seeing in the cockpit at this point.

Thanks for the report and analysis, hell I always thought pilots knew their altitude down to 3-5 feet wasn't aware there were so iffy in the cock pit or from ATC. 

Still, that sounds like a pilot error and perhaps ATC could have been more specific or asked more questions.

[Zaphod58]

Thanks for the report and analysis, hell I always thought pilots knew their altitude down to 3-5 feet wasn't aware there were so iffy in the cock pit or from ATC. 

Still, that sounds like a pilot error and perhaps ATC could have been more specific or asked more questions.

There were multiple errors that will be found, but the primary errors were on the Blackhawk crew.

Altimeters are only reliable within a set limit. A 50 foot error isn’t uncommon, but to have both that far off from each other indicates one was set wrong, or something was wrong with it. Far more likely that it was set wrong.

[DontTreadOnMe]

There were multiple errors that will be found, but the primary errors were on the Blackhawk crew.

Altimeters are only reliable within a set limit. A 50 foot error isn’t uncommon, but to have both that far off from each other indicates one was set wrong, or something was wrong with it. Far more likely that it was set wrong.

How often are they checked for accuracy?

[Zaphod58]

How often are they checked for accuracy?

Generally every heavy maintenance period the instruments are pulled and over hauled, unless they’re written up first. I’m not sure what the period is for the UH-60, but for Air Force planes it’s 5-7 years between PDMs(heavy maintenance periods). There are unit level inspections between PDM periods as well that might catch any issues.

[Zaphod58]

The NTSB issued two recommendations to the FAA as part of their preliminary report. The first would prohibit helicopter operations on Route 4 from Hains Point and Wilson Bridge when 15 and 33 are in use at DCA, while the second would designate an alternate route between those two points when Route 4 is closed off for DCA operations.