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Proposed AD for PA-28 wing spars

The reason that FAA came up with such a complex formula is simply that the US system has a very obvious fault when it comes to logging: Number of landings are not logged – at least not for small GA aircraft.
For issues (like this one) where the by far most relevant factor is exactly the number of landings, they have a significant challenge to determine a good rule for applicability of the AD.
So someone came up with "as we don’t have the number of landings, let’s use the number of flight training hours as flight training typically involves many (and hard) landings).
Unfortunately: Number of flight training hours are also not logged for an aircraft.

Therefore they had to come up with such a weird formula that is basically taking the number of 100h inspections (but only the 91.409(b) ones) as proxy for flight training hours as proxy for landings.

To apply this in EASA-land, there need to be substantial adjustment (as we don’t have 91.409(b) 100h inspections) but will be much simpler as we log number of landings for our airframes.

Germany

Malibuflyer wrote:

Number of landings are not logged – at least not for small GA aircraft.

Although the FARs do not require that Landings are recorded, all the FAA Logbooks I have used over the last 20 years have had clear columns for Landings Day/Night, and another Column for Instrument Approaches.
Without these, how do you keep the 3 Landing’s Record for currency in the last 90 days, and/or the 3 Landings required for Night Currency; as well as the 6/6 Instrument Procedures for IR currency?

Rochester, UK, United Kingdom

Indeed; takeoffs and landings logged. IAPs logged in a separate column however. This is the common Jepp US-style logbook; I don’t know what logbook format US pilots actually use.

I also think that landings don’t necessarily stress the airframe. It is hard landings And I bet there are fewer hard landings among say the US owner community than among the European owner community – simply because they have loads of long hard runways, whereas Europe is full of short ones and all sorts of grass etc ones of widely varying roughness.

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

I don’t know what logbook format US pilots actually use.

Nobody I have ever met in the US keeps an aircraft log tracking individual flights on any light aircraft with a time recording device, either tach time or Hobbs time. Maintenance of light fixed wing FAA-certified aircraft is not in general done based on cycles or anything else except condition or (worst case) by hours… these are not over refined spaceships, they are intended for practical daily use like a car, until they reach their defined lifetime as recorded in hours or simply because failures start to occur at some huge number of hours not defined in certification. Then if the will exists to extend life past the practical limit, things including ADs inevitably get less practical.

On an antique aircraft that does not have an hours recording device, a flight log is legally required by FAA for the obvious reason of keeping track of time in service for any time based inspections, oil changes etc.

I can figure out the details of what my (modern-ish 1971) plane has been doing since I owned it, even if I can’t remember, because I’m the only one flying it and I can check my pilot logbook. The maintenance logs do not provide any details, only the aircraft or engine time in service on the date of a given maintenance operation.

Last Edited by Silvaire at 17 Jan 18:18

Another useful video on NDT specific to the AD



Oxford (EGTK), United Kingdom

Peter_G wrote:

Although the FARs do not require that Landings are recorded, all the FAA Logbooks I have used over the last 20 years have had clear columns for Landings Day/Night, and another Column for Instrument Approaches.

Could it be, that you are mixing up Airplane and pilots logs? What would be the reason to log number of instrument approaches for an airframe?

Peter wrote:

I also think that landings don’t necessarily stress the airframe. It is hard landings

While hard landings stress it more, every landing stresses the airframe – that is exactly the reason why in the world of heavy metal number of landings (cycles) are also tracked/logged and some lifetime limits are relating to these.
Plus: Obviously number of landings is a much better proxy for number of hard landings than flight time….

Peter wrote:

simply because they have loads of long hard runways, whereas Europe is full of short ones and all sorts of grass etc ones of widely varying roughness.

That’s actually an interesting side topic: If it is true what you wrote just some lines above, that landings don’t matter but only hard landings, one could argue that grass is much better for the airframe, as a somehow soft grass absorbs much of the initial hit of a hard landing and therefore even if the ground is somehow rough, the max G on landing should be softer than on hard surface.

Last Edited by Malibuflyer at 18 Jan 07:40
Germany

Malibuflyer wrote:

every landing stresses the airframe – that is exactly the reason why in the world of heavy metal number of landings (cycles) are also tracked/logged and some lifetime limits are relating to these.

I thought it was more related to pressurisation/depressurisation cycles, actually.

ELLX

Malibuflyer wrote:

While hard landings stress it more, every landing stresses the airframe – that is exactly the reason why in the world of heavy metal number of landings (cycles) are also tracked/logged and some lifetime limits are relating to these.

While I agree than landings stresses the airframe, I thought that the major reason for cycle limits in transport category aircraft are stresses and eventual metal fatigue caused by pressurisation and depressurisation.

ESKC (Uppsala/Sundbro), Sweden

Airborne_Again wrote:

I thought that the major reason for cycle limits in transport category aircraft are stresses and eventual metal fatigue caused by pressurisation and depressurisation.

As to my knowledge, some of the limits for wheels, landing gear and brakes are also defined in full cycles – and they might not that much be affected by pressurization ….

Germany

Most GA planes were designed to be simple, and not have time/cycle limited airframe components. A can think of a few exceptions, Piper Tomahawk, Seminole, and PZL Wilga come to mind, but generally the airframes are “On condition” unless later action changes that (AD). For this reason, there is little need to track landings (airframe cycles). The Twin Otter, for example is both hour and cycle limited for specified airframe components. The manufacturer’s ratio is two cycles per hour, and there is a requirement to consult the manufacturer if the usage is a lot different from that. The wing boxes, wing struts, and two fuselage frame hour and cycle out, and must be replaced – it’s a big job!

Landings of a simple GA plane may not be a source of unusual loading compared to flight loads. Yes, A Cherokee does impose wing spar loads during a landing, but, bear in mind that a decent landing may be 1.2G or so, you could easily subject the spar cyclically to that for hours in flight. When you land, the weight of the plane is being supported by the spars (you’re flying), and if you ease it onto the ground nicely, the spars would see little change in loading the whole time, the inboard spar does not know if air, or wheels are supporting a Cherokee’s weight.

AD’s constitute an “airworthiness limitation” which is imposed after the design is certified. If the airframe has hour/cycle/life limits as a part of its type design, those will be airworthiness limitations imposed in the approved maintenance manual. manufacturer’s don’t always get this right at design, and the authority at approval, sometime there are new factors which only time in service show. As we learn more about aging airframes, there will be more airworthiness limitations imposed – by AD…

Home runway, in central Ontario, Canada, Canada
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