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According to this article, the Dash 8 has a "maximum altitude for safe retraction".

This wording implies that there is some danger of retracting the gear above a certain altitude. Is this the case, or was this just sloppy language and the danger is actually in flying above a certain altitude with the gear extended?

In either case, why does this safety limit exist?

Specifically:

  • What are the engineering reasons for this limitation?
  • What are the potential consequences of retracting the landing gear above this altitude (or flying too high with the gear extended)?
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  • $\begingroup$ I feel that all three answers to date add constructively to the overall answer, and I have +1'd all of them. I'd be happy giving any of them the green tick, so I'll just choose the earliest. $\endgroup$ Commented Apr 4, 2022 at 1:21

3 Answers 3

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You can find the actual ATSB report here and that information was pulled right from it, they are not mis-types:

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The report goes on to state (bolded for emphasis):

The operator advised that, although the maximum landing gear altitude limit was exceeded, the aircraft maintenance manual did not require an inspection of the landing gear. The manufacturer confirmed that an inspection of the landing gear was not required. It stated that the 15,000 ft altitude limit was a practical value, not a limiting value. It was based on the normal envelope of landing gear operation. It advised that the aircraft had been demonstrated above 15,000 ft with the gear extended and confirmed the gear would operate normally above 15,000 ft. As such, no inspection was required.

The note likely landed in the manual due to the fact that gear can effect stall characteristics. Typically gear deployment and retraction limits are speed based in my experience.

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    $\begingroup$ The first paragraph of this answer doesn't seem right. You write "those quotes were indeed pulled right from it," which made me think that the phrase "maximum altitude for safe retraction" is a direct quote from the ATSB report, and therefore accurate. In fact, the opposite is true. $\endgroup$ Commented Apr 1, 2022 at 14:52
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    $\begingroup$ Is the idea that maintaining an altitude over 15,000 feet would generally require an airspeed of over 205 knots, which wold preclude operation of the gear, but that if e.g. the plane was at a nose-down pitch that was insufficient to maintain altitude but allowed airspeed to be reduced below 205 knots, then the gear could be operated regardless of altitude? $\endgroup$
    – supercat
    Commented Apr 1, 2022 at 16:47
  • $\begingroup$ @supercat The report actually states: "A review of the recorded data for the flight confirmed that neither of the landing gear speed limits were exceeded." $\endgroup$ Commented Apr 1, 2022 at 19:58
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This wording implies that there is some danger of retracting the gear above a certain altitude. Is this the case, or was this just sloppy language and the danger is actually in flying above a certain altitude with the gear extended?

It appears that the article is indeed sloppy in its use of the phrase "maximum altitude for safe retraction". Later the article states "However, this was above the maximum altitude at which landing gear could remain extended, at 15,000 feet,".

If I were a pilot in an unusual situation, my likely actions would be very different if I thought I had exceeded the "maximum altitude for safe retraction", versus if I thought I had exceeded the "maximum altitude at which landing gear could remain extended".

Another answer has pointed out the reasons why the manual contains a statement about the maximum altitude at which landing gear could remain extended. It appears that this pertains solely to what had been previously demonstrated in actual flight. It seems unlikely that, for a given IAS, there would be any real problems associated with flying with the landing gear extended above the altitude limit that had actually been demonstrated in flight.

Though since flutter is a TAS-dependent phenomenon, and TAS (for a given IAS) increases with increasing altitude, and likelihood of flutter is also dependent on aircraft configuration, you never know. Once you venture beyond the realm of what has been actually been demonstrated, you become a test pilot. Not recommended when flying with passengers.

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  • $\begingroup$ Any chance that extra cold air getting into the gear wells could be a problem? (vs. the smaller amount of cold air with the gear up)? I'm guessing not, since there's no relevant heat source nearby that would keep anything above ambient temp with the gear up; not like that's airtight and insulated. And presumably the moving parts don't have a problem with the cold, or it would be a retract/extend limit, not a gear-out limit. $\endgroup$ Commented Apr 3, 2022 at 1:41
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I should start with the remark that I have no flight experience or education whatsoever. But after reading the ATSB report I have a plausible theory about the limits.

Generally, limits for operating aircraft in their various configurations have one of two reasons: Exceeding them may damage components, or exceeding them leaves the proven safe flight envelope of the aircraft.

The listing of the "Relevant aircraft limits" in the ATSB report mixes limits from both categories:

  • The speed limit for gear operation exists because the forces on the landing gear can damage the gear or the hydraulics. It is lower than the maximum speed with lowered and locked gear because the gear is, unsurprisingly, mechanically more stable when locked. If the landing gear is operated above this speed limit there is a danger of mechanical damage. Therefore an inspection is mandatory.
  • Whether the higher air speed for flying with gear extended indicates a mechanical limit or would lead to unsafe aerodynamics isn't obvious.
  • The altitude limit, by contrast, is almost certainly not a mechanical limit. The aerodynamic forces on the gear are actually lower than at ground level. The manufacturer's response indicates that it was simply an arbitrary upper threshold. That plane configuration was never verified at higher altitudes, because there was no need.1 Because the limit does not indicate a component limit there is no need to inspect the component when it is exceeded.

The article, as quiet flyer already observed, is sloppy and conflates "operating" with "being extended".

The confusion of categories with the mixed listing leaks into the aftermath considerations whether an inspection is necessary. (It is unclear to me from the passage in the ATSB report whether the ATSB initiated this discussion or whether the ATSB merely reports the communication between operator and manufacturer.)


1 Similar to a new medical drug: If it has never been tested on 5-year-olds it cannot be approved for them, even if there is no indication that it would be harmful.

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