Oct. 30, 1935. Leslie Tower, chief test pilot of Boeing, and Major Ployer Peter Hill, an Army test pilot, jumped in the Boeing Model 299 at Wright-Patterson Airfield in Dayton, Ohio. Better known in history as the Boeing B-17 ‘Flying Fortress’, the Model 299 would dramatically change the battlefield during WWII. With his four turbo-supercharged radial engines, giving the Fortress a total of 4.800(!) horsepower and a ceiling of 36.000 feet, it could carry a bomb load of up to 6.000 lb over a distance of 2.000 miles thereby fitting perfectly in the new era of ‘strategic bombing’.
Now, you might be wondering what this classic WWII - era bomber has to do with you… The answer to that question is quite a lot. Let’s go back to the 30th of October 1935, when Leslie Tower and Major Hill were killed flying the Model 299. What could possibly have gone wrong? The investigation following the accident revealed the reason to be terribly trivial and simple – the control lock had been left in place, leading to the crash and subsequent explosion less than one mile from the runway. However, the root cause was not the control lock, but the high level of complexity which is inherent to flying a four engine aircraft (in comparison, the B-17 was to replace the two engine Keystone biplane and Martin B-10).
During this time flying an aircraft was done by memory, a pilot would study all procedures and execute them by heart. This never posed significant problems with his predecessors, but the high level of complexity of the Model 299 made things could (and would) be forgotten. Boeing was confronted with a problem of human capabilities and their restrictions. Simply put; if two highly trained test pilots in ideal conditions could forget something as basic as a control lock, what would be forgotten when the aircraft was operated by a normal Army pilot?
The answer of Boeing, although simple in nature, revolutionized aviation to this day… the checklist was born. And although considered to be an insult to their intelligence at first, the test pilots of Boeing quickly realized that a step-by-step list of critical actions, quickly improved safety while increasing efficiency as well.
Said otherwise, they were able to perform tasks more quickly - without forgetting - while having more time to actually fly the plane. So what began as an insult became a standard throughout aviation. Referring to the FAA Risk Management Handbook, checklists are considered as ‘an essential flight deck resource’, used to verify the correct setting and operation of the aircraft instruments and systems and ensuring proper procedures are performed during all phases of flight (both normal and non – normal).
A correct usage of a checklist, combined with a good knowledge of the aircraft systems and equipment, will be the difference between making a good decision or a possibly fatal error.
The Cessna 172, once considered to be a quite basic aircraft, has been produced since 1955 and subsequently became a highly developed aircraft. The G1000 cockpit can no longer be considered a "simple" cockpit, comparing this to the basic six instrument panel. Do you see any parallels with the story of Leslie Tower?
Let’s take a closer look at our Cessna 172 checklist, and how a good knowledge of the aircraft systems and proper application of the checklist will help you identify problems before they become critical. Below you may find a practical example of how the increased complexity of the Cessna 172SP systems requires both a good checklist and general knowledge to properly identify any issues at hand.
Step 7 – Bus E Volts (MIN 24V), and step 8 – Bus M Volts (MAX 1,5V), are easy right? Just look at the indicators and confirm the numbers… In essence, that is correct. But what do these numbers mean? Or simply put; what do we check?
Remember the condition for these checks are that the Master Switch is OFF (step 12), basically meaning that the main battery is not supplying any electrical power. The only action taken before is testing and subsequently arming the stand-by battery (step 3). Looking at the simplified electrical schematic of our Cessna 172, and more specifically the position of the stand-by battery, solves most of the questions posed above.
As we can see on the schematic, our stand-by battery is connected directly to the Essential bus feeding the – what’s in a name – essentials (Primary Flight Display, Air Data computer, Engine and Airframe Unit, …). Although connected to the Electrical Bus N°1 and 2° as well, the diodes between theses busses and the Essential bus ensure that the main electrical system (main battery and alternator) can feed the essentials (and by extension charge the stand-by battery) but not vice versa. What does this mean practically? When you experience a full (main) electrical failure (both loss of the alternator and a depleted main battery) the stand-by battery will power your Essentials, and only your Essentials, for +/- 30 minutes. The ‘one-way street’, created by the diodes, prevents in this case that power is waisted to what Cessna considers to be non-essential items.
Going back to the checklist, the checks performed on the electrical system suddenly make a lot more sense. By arming the stand-by battery (and not switching the Master ON) we make sure the Essential bus is isolated or in other words, the ‘one-way street’ signs are still up. That is why we are looking to see a minimum of 24 volts on the Essential bus (meaning the stand-by battery is fully charged and ready to supply the Essentials for 30’) and the diodes are doing their job not letting power leak to the main busses (… MAX 1,5 volts).
As you can see, what can be a considered a simple read and confirm check, becomes quite interesting with some basic system knowledge. Also, this basic knowledge will help you identify and analyse the nature of your problem in case of a non-normal condition.
Example: in flight you are confronted with an alternator failure. Let’s look what the non-normal checklist writes;
As you can see above, in case of loss of alternator power (and unable to restore the power), Cessna will advise you to switch off the Master switch (ALT ONLY) and thereby depleting the main battery first. As we know are electrical system, we now know that by doing this we preserve our stand-by battery power as long as possible. But we also realize now that by doing this, we will not be able to use our flaps with a fully depleted main battery (as these are not connected to the Essential bus)…
I hope I have convinced you about the power of the checklist, and how combined with good system knowledge, it’s a great help in properly identifying troubles before they get critical.
Additionally, never underestimate an item what you might interpret as an ‘insult to your intelligence’ and ‘logical’.
Like checking if the chocks or the control lock are removed. The checklist was designed to assist the pilots in the increasing complexity of the aircraft, not for the ‘increased complexity’ itself. Take your time to properly complete your checklist, and know what you are checking, it might save your life one day!
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