Maximum Landing Impact

[captainpaddy]

Hello!

I know that larger aircraft (medium or heavy aircraft) are required to withstand a landing impact equivalent to a 600 feet per minute rate of descent while at their maximum structural landing weight. Does anyone know what the requirements are for light aircraft? Are they the same???

Thanks!

[Mad (Flt) Scientist]

From the current part 23 regs...

§ 23.473 Ground load conditions and assumptions.

(a) The ground load requirements of this subpart must be complied with at the design maximum weight ......

(d) The selected limit vertical inertia load factor at the center of gravity of the airplane for the ground load conditions prescribed in this subpart may not be less than that which would be obtained when landing with a descent velocity (V), in feet per second, equal to 4.4 (W/S) 1/4, except that this velocity need not be more than 10 feet per second and may not be less than seven feet per second.

Looks like the worst case requirement is a 10fps (=600fpm) landing; the equivalent part 25 text is somewhat different, but still ends up calling for 10fps at MLW.

[captainpaddy]

a descent velocity (V), in feet per second, equal to 4.4 (W/S) 1/4, except that this velocity need not be more than 10 feet per second and may not be less than seven feet per second.


Thanks very much Mad man (or woman). Do you know what this bit means though? It seems to say that the rate could be between 10 and 7 fps but will can be limited to either if the formula gives a value outside the range. What are W and S in the formula?

Anyone?????

[Mad (Flt) Scientist]

Wing loading (Weight divided by wing area) I believe. (I'm not a structures expert, and don't work on part 23 either, so I'm pretty much reading and interpreting the regs as they are - and that can be a dangerous game sometimes, because often there's a mass of interpretative material involved)

[A-3TWENTY]

In the company I am flying for there was a hard landing reported with 2.1G . What´s the equivalence in FPM ?

Cheers

[chornedsnorkack]

In the company I am flying for there was a hard landing reported with 2.1G . What´s the equivalence in FPM ?
Cheers
There is not, and cannot be, equivalence between g-s and fpm.

The reason is the following: a plane touching down at some fpm has to be decelerated to zero rate of descent resting/rolling on the ground (here omitting the case where the rate of descent is nonzero when rolling on a sloping surface). The rate of descent is not stopped instantly (this would require infinite force) - it is stopped over extended timespan, while the aircraft continues descending at a decreasing rate - and the length of the landing gear is decreasing.

If the landing gear does not travel a long distance, low descent rates would generate substantial g forces.

[Doors to Automatic]

But for a standard airliner with standard landing gear (all airliner landing gear has a degree of "travel") there must be a rough relationship between FPM impact and Gs?

[Mad (Flt) Scientist]

Physically, there is a relationship. Provided everything else is constant, of course.

But if you're talking about the MEASURED 'g' and the MEASURED descent rate, then things get a bit more complex.

Generally, you'll be looking at the FDR/QAR data to determine if its a 'hard landing'. Both 'g' and descent rate (or rad alt, which is probably what's being used to determine the latter) are only recorded at discrete intervals - almost certainly not simultaneously. And the impact is of course a very short duration event.

So its very easy to miss the peak 'g', or to not get any data right at the instant of impact. In such circumstances, there's no guarantee of a correlation between the measured parameters.

[chornedsnorkack]

But if you're talking about the MEASURED 'g' and the MEASURED descent rate, then things get a bit more complex.
Generally, you'll be looking at the FDR/QAR data to determine if its a 'hard landing'. Both 'g' and descent rate (or rad alt, which is probably what's being used to determine the latter) are only recorded at discrete intervals - almost certainly not simultaneously. And the impact is of course a very short duration event.
So its very easy to miss the peak 'g', or to not get any data right at the instant of impact.

Can someone explain just why the FDR g measuring instruments cannot be made to record g at peak, because of the peak - rather than at or in addition to regular intervals ?

[Mad (Flt) Scientist]

Because there's a finite bandwidth allocated to the FDR data system, and because it's a digital system (these days).

Because it's digital, it can only take data at discrete points in time; because there is a limited bandwidth, the points in time cannot be very frequent.

It's not uncommon to see quite important flight data parameters recorded at relatively low data rates - like one or two Hertz. Even something like normal acceleration, which we're discussing here, might only be at 8Hz. That means that any event lasting of the order of 0.1sec could be completely missed.

*In addition to the limitations on the (D)FDR side for data rates, you can run up against issues with your onboard computing/databus, if there's anything low rate in the line upstream of the FDR, it can end up basically filtering the data.

* What you're asking for is asynchronous data recording. That implies you have spare bandwidth to start with, or that you're going to discard something else to record the 'extra' data. Choosing what to discard would be troublesome.