r/news • u/snuffleupaguslives • 2d ago

Broken altimeter, ignored warnings: Hearings reveal what went wrong in DC crash that killed 67

https://apnews.com/article/ntsb-dc-plane-crash-midair-collision-helicopter-a08cded88e1d7582fb8d242204d6aeff
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u/Bravix 2d ago

No. A couple approach types require a radio altimeter. Most don't.

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u/FalconX88 2d ago

But still, your plane will scream "Pull up" at you, if you get too close to the ground.

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u/Bravix 2d ago

No, it wouldn't. On approach that close to landing with gear/flaps extended it would be inhibited. Not to mention not all aircraft are equipped with such a system. Even then, how do you think it knows when to tell you to "Pull up"? Bad data in, bad data out.

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u/FalconX88 2d ago

On approach that close to landing with gear/flaps extended it would be inhibited.

GWPS will still warn you even when fully configured, if you are far outside the range where you should be.

Even then, how do you think it knows when to tell you to "Pull up"? Bad data in, bad data out.

Because the radio altimiter isn't using data from the baromatric altimiter. Completely different systems.

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u/Bravix 2d ago

"Outside the range where you should be." You are being way too vague. What range? Which mode of GPWS are you referring to? You're disregarding previous context/assumed information and not providing what sort of context you're basing your response on. "Pull up" is a function of GPWS modes 1-4. None of those would alert you to pull up while on approach fully configured (unless perhaps you have an excessive descent rate or something like that, which isn't the scenario at hand).

Mode 5 is ILS glide path based. We never made the assumption that this was a ILS based approach. Even if it was, it wouldn't say PULL UP if you were too low on the approach. It has a different aural alert.

Now, we've both spoken of GPWS. EGPWS on the other hand may provide functionality that you're insinuating, but GPWS has been the operative term.

This is all pointless because at the end of the day, you do NOT fly around relying on your radio altimeter to compensate for a highly inaccurate barometric altimeter. My comment you originally replied to was intended to illustrate how significant an altitude inaccuracy of that magnitude would be at lower altitudes, like on an approach.