Data from Air Accident Show Conflicting Altitude Readings

More from the article...

... Investigators also said that about a second before impact, the jet's flight recorder showed a change in its pitch. But they did not say whether that change in angle meant that pilots were trying to perform an evasive maneuver to avoid the crash.

Data from the jet's flight recorder showed its altitude as 325 feet (99 meters), plus or minus 25 feet (7.6 meters), when the crash happened Wednesday night, National Transportation Safety Board officials told reporters. Data in the control tower, though, showed the Black Hawk helicopter at 200 feet (61 meters) at the time.

The roughly 100-foot (30-meter) discrepancy has yet to be explained.

Investigators hope to reconcile the altitude differences with data from the helicopter's black box, which is taking more time to retrieve because it became waterlogged after it plunged into the Potomac River. They also said they plan to refine the tower data, which can be less reliable. ...

@#2

I do not disagree at this point.

But until the more detailed data arrives, I have another question ...

Did the tower data show the aircraft was also at 200 feet, i.e., at the same altitude the tower thought of the helicopter?

@#4 ... I saw radar plot images showing the CRJ at 400 feet and the UH-60 at 200 feet and then 2 seconds later showed them both at 300 feet. ...

I have read reports of a last-second maneuver by the aircraft.

What you describe might mean that the aircraft dropped down to the altitude of a rising the helicopter?

It usually takes a while for the black box data to be analyzed and a timeline made.

@#7 ... The CRJ was landing, so it was certainly losing altitude. ...

Yup.

But I wonder if there was a sudden decrease in altitude that is not typical of the normal lowering of altitude during a landing?

@#9

I look forward to the detailed analyses of the black boxes.

Until then, too many questions, not enough answers.

@#11 ... Read the helo CVR was waterlogged when they found it. ...

Supposedly, they are designed for such an outcome. Idoes, however, take loger to analyze the data should that occur, as you note.

Flight recorder - Specifications
en.wikipedia.org

... The design of today's FDR is governed by the internationally recognized standards and recommended practices relating to flight recorders which are contained in ICAO Annex 6 which makes reference to industry crashworthiness and fire protection specifications such as those to be found in the European Organisation for Civil Aviation Equipment[45] documents EUROCAE ED55, ED56 Fiken A and ED112 (Minimum Operational Performance Specification for Crash Protected Airborne Recorder Systems).

In the United States, the Federal Aviation Administration (FAA) regulates all aspects of US aviation, and cites design requirements in their Technical Standard Order,[46] based on the EUROCAE documents (as do the aviation authorities of many other countries).

Currently, EUROCAE specifies that a recorder must be able to withstand an acceleration of 3400 g (33 km/s2) for 6.5 milliseconds. This is roughly equivalent to an impact velocity of 270 knots (310 mph; 500 km/h) and a deceleration or crushing distance of 45 cm (18 in).[47] Additionally, there are requirements for penetration resistance, static crush, high and low temperature fires, deep sea pressure, sea water immersion, and fluid immersion.

EUROCAE ED-112 (Minimum Operational Performance Specification for Crash Protected Airborne Recorder Systems) defines the minimum specification to be met for all aircraft requiring flight recorders for recording of flight data, cockpit audio, images and CNS / ATM digital messages and used for investigations of accidents or incidents.[48] When issued in March 2003, ED-112 superseded previous ED-55 and ED-56A that were separate specifications for FDR and CVR. FAA TSOs for FDR and CVR reference ED-112 for characteristics common to both types.

In order to facilitate recovery of the recorder from an aircraft accident site, they are required to be coloured bright yellow or orange with reflective surfaces. All are lettered "Flight recorder do not open" on one side in English and "Enregistreur de vol ne pas ouvrir" in French on the other side. To assist recovery from submerged sites they must be equipped with an underwater locator beacon which is automatically activated in the event of an accident. ...

Lots of detail in that article.

@#15

Yeah.

The data on current black boxes (even though they are yellow or orange for visibility) seems to be stored on memory chips, not a disk drive.

Those memory chips may need to be removed from the water-corrupted environment of the black-box, and placed into an environment that allows them to be read.

That may be what takes the additional time in occurrences such as this one.