# What scientific evidence is there about the effectiveness of pre-flight walk-arounds?

As a student pilot, often on my walk-around, I get the impression that there are any number of hazards that could pass me by, simply because I can look at every part on the checklist but I wouldn't necessarily recognise a problem with that part. This idea seems more plausible when I speak to more experienced pilots of the same type, and each of them warns me about checking one thing in particular that's not on the checklist, because they once saw an accident that was caused by a failure there - and crucially, each of them warns me of a different thing.

At the same time, when I read the monthly accident report bulletins, there are several every month where the cause was some part failing in flight, and they either imply that the failure should have been caught in a pre-flight inspection, or they explain why it might not have been visible. In one local incident a few months ago, a pilot made a hard landing and thought he made an extra-careful inspection afterwards, but made a subsequent flight without noticing that both blades of the prop had been shortened and the firewall was crumpled. More generally, taking off with your pitot cover still in place is the stereotypical example of a bad inspection.

Pre-flight inspections seem very error-prone, on the face of it. What evidence is there about how error-prone they are, and what fraction of defects they detect? I'm asking for either scientific experiments under controlled conditions, or statistical analyses of aircraft grounded after unsatisfactory inspections vs. failures in flight. Turning the question around, you might instead answer about how much less safe we would be in GA or air transport if we didn't have pre-flight inspections at all. This related question is pretty relevant, but focuses on checklist items which are forgotten completely: I'm hoping for a more general answer which also includes defects that are not detected even if the checklist is performed correctly.

• They aren't meant to detect things like structural failure, but you should be able to notice missing bolts/nuts/safety wires, loose/missing fuel caps, flat tires, pitot tube covers, clogged/blocked static ports, blade damage, structural damage, incorrect fuel, worn brakes/tires, missing/broken antennas, etc. that would keep you from conducting a safe flight. – Ron Beyer Jun 17 '16 at 13:34
• I doubt that you'll find scientific experiments under controlled conditions. It's rather like asking is there any proof that looking both ways before crossing the road actually reduces accidents. – Simon Jun 17 '16 at 13:43
• Doing a walkaround is more effective than not doing one. Do you need a scientific study to tell you that? – abelenky Jun 17 '16 at 14:30
• Every time a pre-flight detects something the matter with a plane, that's evidence of the effectiveness of pre-flights. But that won't result in an incident being reported. – DJClayworth Jun 17 '16 at 15:13
• @abelenky It would be nice to have something more to go on than a flat assertion. – Dan Hulme Jun 17 '16 at 15:53

First off you are required by the FAR's to preform a preflight check,

Sec. 91.7

Civil aircraft airworthiness.

(a) No person may operate a civil aircraft unless it is in an airworthy condition.

(b) The pilot in command of a civil aircraft is responsible for determining whether that aircraft is in condition for safe flight. The pilot in command shall discontinue the flight when unairworthy mechanical, electrical, or structural conditions occur.

Don't forget the Preflight is also about making sure the aircraft is legally airworthy (VFR or IFR minimum equipment) as well as being physically airworthy.

Here is a brief from the FAA on the matter. It has at least one example of an accident that occurred as a result of a bad preflight. According to this Flying Magazine Article 15 accidents have been the result of a bad preflight in the past 10 years (from the publish date of that article). Here is another example as well. Clearly it has lead to accidents in the past but you may have an issue finding any data on times that it has found things and potentially saved lives (as that is often not reported to anyone but the mechanic).

Nasa conducted a study on checklist usage (preflight included) which is along the lines of what you may be looking for.

Here is another FAA article on preflights and has some more specific stats on what you are looking for.

Consider this: NTSB aviation accident data show that in the last 10 years, poor preflight inspections caused or contributed to 156 general aviation accidents and resulted in 41 fatalities.

You can compare that to the total number of other GA accidents for an approximation of how important it is in causing a crash. of When it comes to the FAA a great deal of analysis is carried out on accident data not necessarily preemptively. One of the other analysis issues here as mentioned in the comments is much of the reasons that planes get grounded does not get reported to any official authority (aside from the mechanic) so the data may not be public or easily accessible.

If we include preflight checklists and walk arounds in the same category. Following the Northwest 255 accident that was the result of misunderstanding/misusing pre-flight checklists the NTSB did do a study on preflight (and all checklist) procedures. You can find the study and results report here which includes some stats.

This idea seems more plausible when I speak to more experienced pilots of the same type, and each of them warns me about checking one thing in particular that's not on the checklist, because they once saw an accident that was caused by a failure there - and crucially, each of them warns me of a different thing.

This may be more a case of correlation over causation. If you are involved in aviation long enough (especially the older generation that has flown when laws and planes were much different) you see lots of things. Some of the issues they talk about may have been covered by AD's since they encountered them (if you are flying a common airframe like a 172 or PA-28).

As for not finding stuff this will come with how well you know your plane. As you start to fly the plane more and more (you should be training in one plane or at least only one type of plane). Eventually you will begin to know whats where, the POH can help with that, and you will be able to recognize if something is out of place.

I can speak from personal experience that I have delayed many flights due to things I have found on the preflight. I had a cracked cowling bracket, countless low gear struts that needed air, many fuel calls in the rental planes, a fuel vent issue, and one breaks issue.

Of course you will always have human factors present in manned flight.

• While this is a good opinion (and I tend to agree with it), the OP specifically asked for verifiable statistical data on the topic. I don't see any here so...this doesn't really answer the question. – Jay Carr Jun 17 '16 at 17:21
• The Flying Mag article quotes some hard stats on accidents caused by preflight issues I am still looking for harder numbers but there is no study I have yet to find. – Dave Jun 17 '16 at 17:58
• Also, citing accidents caused by poor preflight inspections isn't useful (for this question) unless those inspections were identified as poor before the accidents -- and unless we generally know how many poor inspections did not result in accidents. Or, another way to say that is: maybe preflight inspections are by their nature poor, but the only ones that are identified as such are the ones after which something went wrong that might have been caught by a hypothetical, unrealistically good inspection. – yshavit Jun 17 '16 at 18:56
• I was trying to provide data that was as relevant to the question and has been published. From my searching of FAA reports, documents, NTSB summaries and articles this is the best I was able to come up with so far. I will admit that it is not as direct of an answer as I would like but I feel as though its on topic considering the lack of information that seems to be out there on this topic. – Dave Jun 17 '16 at 21:41

This answer does not answer the question. Rather, I'll try to explain why I believe it is unanswerable (on this site at least).

Let $A$ denote having an accident (of any cause), and $I$ denote having a preflight inspection.

This question is basically asking the comparison of two conditional probabilities: $P(A | I)$ and $P(A | \lnot I)$.

Unfortunately nobody reports an event where $\lnot I$ is true, since you're never supposed to skip a preflight inspection.

Another way of looking at it will be, given a proper preflight inspection is done, there can be 3 outcomes:

1. A potential safety hazard was discovered. The flight was grounded. (We never know if it would have caused an accident)
2. No hazard was discovered, but the flight ended with an accident anyway.
3. The flight proceeded safely.

Note that $P(A | I)$ would be (2), while $P(A | \lnot I)$ would be (1) + (2).

However, (1) is not reported. Aircraft maintenance logs are generally not sent to an aviation authority, unless an investigation has opened on that airframe. This precludes scanning a database for (1).

Another word for (1) is how many accidents have been prevented by a walk around.

Airlines can reasonably have this data. They would know how many mechanic inspections are called because their pilots found something unsatisfactory in their walk-around. However I doubt such data is ever published (except for occasional news report), after all, airline pilots are not supposed to find something during their walk-around. If they do, it shows the airline's maintenance is poor.

• Your question presupposes that nobody ever admits a real or perceived maintenance shortcoming because they are more concerned about looking bad than about the safety outcomes. However, I often read accident and airprox reports about such incidents, so I find your assumption unconvincing. Second, there are several data gathering and statistical techniques for finding out the truth even when people don't want to admit to embarrassing truths. These have been used for finding the prevalence of other things, for example illicit use of anti-depressants by pilots. – Dan Hulme Jun 19 '16 at 7:16