Quick question

[BugBear]

Re : data stored on DFDR. For example, if a controls position is sampled once per second, and the record is reporting "One second" between movements, is the range of interval one second? Say the report reads "01". Does this mean it is somewhere between simultaneous and one second?

Let's say the switches are "Run/Cutoff" switches. If one switch closes at "x" ( per timestamp) and the next co-located switch is "x+1", could the switches have closed simultaneously? A "range" of operations of one second?

Thanks

[Someone Somewhere]

My understanding (from reading some of the raw data in previous FDR reports) is that the sampling is kind-of like old analogue video: there's always something being sampled, and what is being sampled moves in a cycle. So the time between two samples of the same item is constant, but the time between two samples of different things relies knowing their position in the sampling cycle.

E.g. a very simple recorder might record the following:
0.0s Eng 1 cutoff
0.1s Eng 1 N1
0.2s Eng 1 N2
0.3s Eng 2 cutoff
0.4s Eng 2 N1
0.5s Eng 2 N2
0.6s left aileron position
0.7s left elevator position
0.8s rudder position
0.9s stabiliser trim position
1.0s Eng 1 cutoff
1.1s Eng 1 N1
1.2s Eng 1 N2
1.3s Eng 2 cutoff
1.4s Eng 2 N1
1.5s Eng 2 N2
1.6s left aileron position
1.7s left elevator position
1.8s rudder position
1.9s Flaps position

Note that:

• The time between successive measurements of the same thing is always the same
  • In this case, most items are 1 second.
  • Some items, in this case the trim and flaps, are sampled less frequently. The x.9 second slot gets used for one item on even seconds, and the other on odd seconds, so there's a 2-second sampling interval.
• The time between measurements of different items depends on their position in the cycle. Eng 2 cutoff is sampled 0.3 seconds after Eng 1 cutoff, but then it's 0.7 seconds before Eng 1 cutoff is next sampled.
  • I suspect that the order of this sampling isn't strictly defined; it will depend on what the FDR/EAFR is selected to record and software version. I am not sure you can look at a previous report on the same aircraft type and assume everything is the same.
• If both switches were cut simultaneously at 0.2 seconds, the FDR would show Eng 2 being cut first at 0.3 seconds and Eng 1 at 1.0 seconds.
• If both switches were cut simultaneously at 0.7 seconds, the FDR would show Eng 1 being cut first at 1.0 seconds and Eng 2 at 1.3 seconds.

I don't know why the report says '01' seconds. I suspect this is just a typo.

[BugBear]

Hmmm... Thanks S/S

Originally Posted by Someone Somewhere

My understanding (from reading some of the raw data in previous FDR reports) is that the sampling is kind-of like old analogue video: there's always something being sampled, and what is being sampled moves in a cycle. So the time between two samples of the same item is constant, but the time between two samples of different things relies knowing their position in the sampling cycle.

E.g. a very simple recorder might record the following:
0.0s Eng 1 cutoff
0.1s Eng 1 N1
0.2s Eng 1 N2
0.3s Eng 2 cutoff
0.4s Eng 2 N1
0.5s Eng 2 N2
0.6s left aileron position
0.7s left elevator position
0.8s rudder position
0.9s stabiliser trim position
1.0s Eng 1 cutoff
1.1s Eng 1 N1
1.2s Eng 1 N2
1.3s Eng 2 cutoff
1.4s Eng 2 N1
1.5s Eng 2 N2
1.6s left aileron position
1.7s left elevator position
1.8s rudder position
1.9s Flaps position

Note that:

• The time between successive measurements of the same thing is always the same
  • In this case, most items are 1 second.
  • Some items, in this case the trim and flaps, are sampled less frequently. The x.9 second slot gets used for one item on even seconds, and the other on odd seconds, so there's a 2-second sampling interval.
• The time between measurements of different items depends on their position in the cycle. Eng 2 cutoff is sampled 0.3 seconds after Eng 1 cutoff, but then it's 0.7 seconds before Eng 1 cutoff is next sampled.
  • I suspect that the order of this sampling isn't strictly defined; it will depend on what the FDR/EAFR is selected to record and software version. I am not sure you can look at a previous report on the same aircraft type and assume everything is the same.
• If both switches were cut simultaneously at 0.2 seconds, the FDR would show Eng 2 being cut first at 0.3 seconds and Eng 1 at 1.0 seconds.
• If both switches were cut simultaneously at 0.7 seconds, the FDR would show Eng 1 being cut first at 1.0 seconds and Eng 2 at 1.3 seconds.

I don't know why the report says '01' seconds. I suspect this is just a typo.

Most interesting and I have a theory re the "01"

Without a decimal, the number implies a resolution of +/- One. That would be reasonable if it was reporting a movement, and not a "State". A condition....

What if the Cut was not generated by a physical movement, but by a Signal? Say a difference of 100 milliseconds? In theory, a differentiation in timing of a physical motion and a signal being computed in milliseconds? A command direct from software in the EEC to the Fuel MU without being detoured through the RDCs? (Or FADECs?) If an Overboost, wouldn't bits of seconds be crucial?

Thanks again.

[Someone Somewhere]

Switches generate a signal, and that signal transitions in tiny, tiny periods of time - switches aren't like valves, where there's a long period of half open. If you've ever been an idiot and tried balancing light switches half way, you've seen the impacts of this.

Contact bounce is a real issue with practically any sensor or switch feeding into electronics, and it's likely that there's some averaging and smoothing in the RDCs over a handful of milliseconds to handle that.

The EAFRs/FDRs only sample data once a second. That doesn't mean that the rest of the plane isn't operating on millisecond timings, just that the EAFRs only record each parameter (at most) once a second.

IIRC they were reasonably good at disambiguating terms in the report. I don't see them doing all of that then intentionally including "01" with some esoteric meaning only known to their internal flight data group that's only just been set up, and not even adding any kind of note or meaning.

[ignorantAndroid]

Originally Posted by Someone Somewhere

The EAFRs/FDRs only sample data once a second. That doesn't mean that the rest of the plane isn't operating on millisecond timings, just that the EAFRs only record each parameter (at most) once a second.

The regulations specify a minimum sampling rate for each parameter. They range from 8 Hz to 1/64 Hz (i.e. one sample every 64 seconds).

https://www.govinfo.gov/content/pkg/...rt121-appM.pdf

[Someone Somewhere]

Originally Posted by ignorantAndroid

The regulations specify a minimum sampling rate for each parameter. They range from 8 Hz to 1/64 Hz (i.e. one sample every 64 seconds).

https://www.govinfo.gov/content/pkg/...rt121-appM.pdf

Ah, excellent.
It looks like most of the engine controls and indications are 1Hz at most, though.

I was really trying to give a broad-level view of how the sequential sampling operates, rather than be exhaustive. Sampling is obviously a lot higher than 10 parameters per second.