Blast from the past: Venturi-driven vacuum system on a 172

On the grass parking area in front of the Redcliffe Aero Club is an old 172 with a fairly unusual design feature: a dual Venturi-driven vacuum system.

The VH register says that VH-DCO was first registered in Australia in 1962. It is however quite likely that the airplane was first registered in the US and later imported into Australia. The registration data shows the model as a plain “172” and not “172A” or “172B”, which seems to indicate that it was built between 1956 and 1960, according to the Cessna 172 page on Wikipedia.

Another cue is the square tail of the original 172 model, which contrasts with the more familiar swept tail introduced in 1960 with the 172A model. Lots of other details on the original 172 can be found in this pilot report. I learned for example that the rear window was first introduced in 1963 with the 172D model. I personally love this feature of the 172, if only because it makes it easier to check for drift on climbout after take-off by having a quick glance at the runway behind.

Externally, the Venturi vacuum system looks like two brass horns bolted to the right-hand side of the engine cowling. However, instead of being one long cone-shaped tube as with the horns of vintage automobiles, each of the tubes is narrower about one-third of the way down, which is exactly what you want for exploiting the Venturi effect in order to create a vacuum.

As can be seen on the picture, the diameter at the choke point is about half the diameter of the intake end of the tube. Half the diameter means that the cross-section is reduced down by three quarters, and therefore the speed of the air at the narrowest part of the tube is four times the speed at the intake. I am no aerodynamics expert, but I guess we can assume the speed of the air at the intake to be fairly close to the airspeed of the plane.

I’ll spare you the calculation, but using the Venturi formulas given by the irreplaceable Wikipedia and for ISA conditions at sea level (air density of 1.225 kg/m3), such a Venturi device would ideally generate a pressure drop of 243 hPa for a cruise speed of 100 knots. Of course this assumes that the air is a non-compressible fluid, that it flows nicely in a laminar manner down the tube and that there is no friction on the walls of the tube. In addition, one can see on the picture that there is an inside ring near the intake end of the tube, which probably slows down the air a little on entry.

But you get the idea: the suction produced is strong enough to drive the gyros of the Attitude Indicator (AI) and the Directional Gyro (DG). I assume that the gyro of the Turn Indicator is electrically-driven, but I could not confirm that.

Just like with more modern airplanes which use an engine-driven vacuum pump, a suction gauge on the instrument panel tells the pilot that the vacuum system is working correctly (or not, depending). This can be seen on a picture of the instrument panel of G-BSEP, a 1959 Cessna 172 flying in the UK.

Obviously, the main problem with a Venturi-driven vacuum system is that it requires the aircraft to have reached cruise speed for the gyros to spin fast enough to give a reliable indication of either attitude or heading. This means no AI or DG during taxi and climb-out. I can't think this is too much of a problem for VFR pilots, since attitude and heading information are readily available from just looking outside or at the compass.

There is still one thing I am curious about however. How does the vacuum system behave in slow flight with a high angle of attack, such as when coming in to land? Can the pilot see a drop on the suction gauge? And if so, are the gyros still spinning fast enough to be relied on? Again, not having any AI or DG indication at this stage of flight would be nothing dramatic, since VFR pilots have their eyes looking out at the runway by then. I would love to go out and try it out for myself though :-)

On the medical certificate and CASA's cost recovery model

In Australia, you do not need a medical certificate to start learning to fly, but you definitely need one when the time has come to fly on your own.

As a beginning student pilot, it was difficult for me to imagine that my first solo may be just a few months away. Everything was new. The plane liked to behave in seemingly unpredictable manners. There was never enough time in a circuit to perform all the required actions and go through all the checklists, even when flying a 152 that could barely go faster than the end of the white arc with two people onboard.

Even if you see your first solo as a distant point in the future, you want to be ready for that special day when you’ll take off on your own for the first time. Not being able to fly solo because you delayed going to the doctor would be rather sad.

The process for obtaining the medical certificate is not overly complicated. The price is another story, and I’ll come back to that later.

You start by choosing a doctor in the list given by CASA, and show up on the day with your ARN and photo identification. The nice lady at reception will have you fill in a medical questionnaire, after which you’ll see the doctor. Make sure you go for a Class 2 medical certificate. Class 1 certificates are for professional pilots, while Class 3 are for ATC personnel.

Since I did not have any significant medical history, the medical examination was not different from a routine check-up at the doctor. On the topic of ears I mentioned that I am prone to ear infections while swimming and diving. The doctor picked up on that and was very happy to show me the pictures of his last diving trip to Vanuatu.

Speaking of diving, I found this medical examination actually less thorough than the one required for the PADI Open Water Diver course. That’s probably because private pilots do not fly airplanes through water while breathing through a regulator. Both medical examinations put your sense of balance to the test though. I guess that’s because spatial disorientation may occur in pretty much the same way whether you’re (accidentally) flying through clouds as a private pilot or diving in murky water.

The medical examination itself was $170, on top of which CASA happily slaps a $130 “processing fee”, which brings the total cost of the medical to $300.

In-between the SPL and ASIC applications (see my previous post) and the medical certificate, that’s $510 that a student pilot needs to shell out in overhead costs before his first solo. Just to put things in aeronautical perspective, that’s 2.4 hours of dual instruction in a 152.

Now, I understand that the doctor needs to be remunerated for the medical examination, even though one may find $170 a bit much for what amounts in practice to a long visit to the doctor.

The level of CASA fees can be explained by the so-called cost recovery model, which forces CASA to recover costs “for providing regulatory services to the aviation industry “. Actually, I just found out that the processing fee had been reduced from $130 down to $75 as of July 1^st, 2007. That should help a little bit, even if the principle of cost recovery remains in place.

Talking about the cost of training, CASA recently release a comparative study of the cost of flying training in Australia, New Zealand, the US and the UK. The bottom line is that the UK is most expensive, Oz and NZ are comparable, and the US are a bit cheaper. What I also discovered is that administrative costs in the US are near zero, including for the medical, as shown in the diagram below, taken from the survey.

However, CASA does not see this as impacting the viability of flying training organisations in Australia, so there’s unfortunately no sign that Australia would be moving to the American model anytime soon.

Paperwork for student pilots in Australia

My friend Jacob finally decided to start his training towards a PPL at the Redcliffe Aero Club. Congratulations to him, I hope he’ll enjoy the experience as much as I did.

There’s a few tips I came across when I started my training earlier this year; I thought I would share them with other student pilots. Let’s start with paperwork. What I’m going to say applies only to Australia, apologies to overseas readers.

If there’s a name that’s going to come up a lot in your training, and in conversations with fellow pilots, that’s CASA. The Civil Aviation Safety Authority is the regulatory authority for civil aviation in Australia. In that role, they’re the equivalent of the FAA in the US or the DGAC in France. Among other things, they’re the people who will eventually deliver your pilot licence, so that’s one big reason to make sure you get the paperwork right.

The first step is to obtain an ARN number. This Aviation Reference Number is a personal identification number that will be required for any future interaction with CASA. The application process is rather straightforward: just fill in the form, fax it to CASA, and a week or two later a letter will pop up in your mailbox with your ARN number. So far so good.

The next step is to apply for a Student Pilot Licence (SPL) and an ASIC (Aviation Security Identity Card). This can be done in one go using form 639. The process is a tiny bit more involved here since it requires someone official to check your identity and, for non-Australians, your proficiency in the English language.

In Redcliffe the CFI can do that and the club will then lodge the application for you. However, between Rob’s busy schedule and my own, I found it hard to get hold of him for that purpose, so I decided to lodge the application directly with the regional CASA office. The office is located in Hendra, near Brisbane airport, and is pretty much on the way for anyone who drives from Brisbane’s inner suburbs to Redcliffe.

That was a painless process. I had made sure I had all the required documentation with me, and the whole thing took less than 15 minutes. The English proficiency test took the form of a two-minute informal conversation with a CASA person who, on learning that I was French, explained to me how he used to ferry Airbuses from Toulouse in France to Malaysia.

The SPL and the ASIC card turned up in the mail about 3 weeks later. The whole thing was quick and easy, but not cheap: I left CASA with $210 less in my pocket: $65 for the SPL, and $145 for the ASIC. Next step: the medical examination.