There are many active pilots and prospective pilots who harbour the ambition of eventually gaining their endorsement on a jet aircraft. I’ve been in a fortunate position to help numerous pilots earn their jet aircraft endorsement, so let’s discuss the differences between jet and propeller aircraft and how to fulfill that ambition. A lot of budding jet jockeys are often intimidated by two factors – the expense of the training requirements and the performance capability of jet aircraft, and rightly so.
First, let’s take a look at the aircraft. The two major differences between a jet aircraft and a propeller aircraft are:
* the higher speed and momentum, and
* the lack of throttle response when you need it most.
A jet engine is prone to over and under-fueling and in order to remove that responsibility from the pilot, engineers have devised a fuel control unit to keep the engine from surging or compressor stalling. That means that no matter what you do with the throttle the fuel control unit will give the engine only as much fuel as it can safely metabolise. Additionally as you go higher and the air gets thinner the fuel control unit cleverly limits the amount of fuel going to the engine in order to maintain the optimum fuel air ratio mix, this section of the fuel control unit is the barometric fuel control. This wonderful device also limits the amount of fuel it will give to the engine when you panic and slam the throttle full forward in an attempt to extricate yourself from a sticky situation. This is accomplished within the fuel control unit by a section known as the acceleration fuel control and the result is spool up times from idle to full power of around ten seconds, a lot of time to contemplate your predicament. Early jet engines such as in the MiG-15 and Vampire did not have acceleration fuel control and these engines were easily overfueled and often compressor stalled as a result. Once the engine achieved a higher RPM though it could receive as much fuel as the left hand could demand.
A compressor stall occurs in a jet engine whenever the airflow within the engine is seriously disturbed. Since the compressor blades of a jet engine are just small aerofoils they can actually stall, just like a normal wing, this will have a detrimental affect on the engine’s performance and integrity. It will also raise the pilot’s pulse, blood pressure and laundry bill. The causes of compressor stalling range from foreign object ingestion through to back pressure on the turbine (back bit that drives the engine). Ingestion of foreign objects such as birds, bolts and stones disrupt the airflow by damaging some of the compressor blades. It can also lead to catastrophic failure due to the massive imbalance caused by bits and pieces circulating at 13,000 RPM suddenly having different masses to each other (think about a car wheel that is slightly out of balance and the vibration it causes). Over fueling disrupts the airflow by an abrupt non-linear pressure change in the middle of the engine. Back pressure on the turbine is caused by reverse airflow in the exhaust pipe, usually the result of a stall turn or other vertical manoeuvre having gone beyond the capability of the instigator and ending in a tail slide or other reverse direction event.
It can now be appreciated that handling of a jet engine, whilst much more simple than the many levers of a radial/piston engine, still requires a full understanding of how it works and where it can bite you. The biggest challenge is anticipation and thinking ahead of the aircraft, which brings us to the next problem, momentum. The momentum of a jet aircraft is not only brought about by it’s weight (and higher performance) but also the fact that many jet aircraft are, of course, very slippery through the air. There is also the fact that the main propulsion unit is confined within the airframe and does not have the benefit of a large rotating propulsion device in the air flow that can also be used as an effective braking tool when necessary. While most jet aircraft have speed brakes (deployable barbecue plates) their effectiveness reduces as speed reduces, something about that old velocity squared aspect of those aviation formulas that I can’t remember anymore. What does it all mean? Well the bloody thing won’t slow down as well as you might want and so you will need to be able to judge exactly where and when to manipulate the controls in order to remain within your expected/desired flight path.
Everyone knows that a jet aircraft generally has a higher performance capability than most other aircraft, therefore you will need to encourage the grey matter to elevate it’s processing capability in order to maintain situational parity with your aircraft. In other words the increased speeds will easily outstrip your ability to keep up with it and you can find yourself well behind the aircraft very quickly. The approach and landing speeds of jets commonly exceeds the maximum allowable speed (Vne) of many light aircraft and climb rates start around 4,000 to 6,000 feet per minute as opposed to 500 to 1,000 feet per minute for light aircraft.
In the words of Rolls Royce, if you have to ask the question, you can’t afford it mate. The price of performance is obvious at around 650 – 850 litres per hour for the warbird jets getting around today, you can now appreciate where we are going with expense. Combine this with the extra servicing costs of these aircraft brought about by the adjustment of the damn fuel control unit and a myriad of other clever things that jets have, certainly brings the expense thing into careful consideration.
What About You?
If you are still interested then let’s look at how you can best place yourself in a position to join the jet jockey club. If you are beetling around in a C-152/C-182 and the like, you have a long road ahead. I recommend the ‘stepping stone’ approach, you will need to gradually increase the performance capability of the aircraft you are flying, until you are able to competently control something like a T-28 Trojan. These types of aircraft were used extensively by the military as precursor aircraft for jet conversions because of their performance capability. Whist other aircraft like the Winjeel or Yak-52 might get you there you can reasonably expect to need more hours on them before contemplating a jet conversion.
Let’s look at how many hours you might need. Notwithstanding an individual’s ability level, it could very well be that you are going to need about 30 – 50 hours on one of these higher performance aircraft before you are ready to move up. Of course the more hours you have in the total column of your logbook the better, but a general guide is around 500 hours total. For those who think that is a bit much, it is better than finding out that you need a whole lot more hours on that expensive jet before your instructor will let you loose on it. Depending on your experience and ability you can expect to require anywhere between 10 to even 50 hours of training to achieve the required competency levels on a jet.
A major part of aviation is decision making. While flying any aircraft a pilot is confronted with many and varied situations where good decision making is required to resolve all of the challenges that present themselves. Generally speaking the more experience a pilot has the more likely he/she is to make good decisions. The enemy of decision making is “Old Father Time” and it can be easily seen that the less time you have to make a good decision the more likely you are to come up with the wrong one especially if you are inexperienced. With the performance of a jet in mind the time compression problem is easy to comprehend and, at times, somewhat difficult to overcome. In training for fighter pilots, instructors encourage ‘compartmentalising’ tasks and thinking/planning ahead for the transition into the next task as it develops. For example concentrating on the takeoff roll and likely problems and when it is clear that that task will be successful planning the after take off tasking e.g. checks, trim, attitude that sort of sequence, this then is followed by the climb tasking and the level off cruise setup tasking and so on. It can be seen how these separate tasks are segmented into compartments. For fighter pilots these tasks become very complex as they enter into combat scenarios. For the budding jet jockey this skill set will need to be mastered in order to attain the required ability for successful completion of a jet endorsement.
The most common jet in the Australian warbird fraternity today is the L-39 Albatros. There are some others around like the Jet Provost and S-211 but they are few and far between. The L-39 is a relatively simple aircraft and doesn’t require external power for starting as it has an auxiliary power unit or APU. Being a Soviet style design built in Czechoslovakia it is amazingly robust and reliable however, it does have certain places in the flight envelope that will bite you hard. The wing of an L-39 is a super-critical aerofoil (reach for advanced aerodynamic manual) and as such it has a very different drag curve to conventional wings. It falls off the back of the drag curve swiftly yet subtly and when combined with a spool up time of 9 – 11 seconds it can be seen that on final approach this is a typical ‘dead man’s curve’ in the envelope i.e. you can arrive in a situation from which you will not recover. Additionally the aircraft suffers significantly from interference drag that occurs when the undercarriage is down requiring a lot of power (around 90%) to compensate. In the glide this can be easily demonstrated as the rate of descent doubles with the gear down, so in a forced landing ‘gear down’ is taken on very short final. All aircraft have their idiosyncrasies and jets are no exception so in your training you will need to be particularly cognisant of them and learn to avoid them.
The standard of training you receive naturally depends on who you get to do your instruction. Most of the guys I know who instruct on these jets uphold a pretty good standard but there are a couple of dodgy ones too. Let’s face it, nobody who owns these expensive machines is going to let a dill use their aircraft. What you need to realise is that to be a competent jet pilot requires a high level of self-discipline, near enough is not good enough. If you don’t have pride in your ability to fly an aircraft as precisely as you can then you are going to struggle in a jet. Contrary to what you might think, the ability to fly smoothly is the hallmark of a good jet pilot as this preserves valuable energy. Once you have shown competency in handling the aircraft in normal phases of flight you will move on to the emergency side of your training which will test you out considerably. Typical forced landing patterns use a high key (first aiming window for success) of 3,500 feet and a low key (last aim point for success) of around 1,500 – 2,000 feet.
In the end it is a judgement exercise that you will need to get very good at because the ejection seats are not armed in all of the jet warbirds operating today (Temora Aviation Museum jets excluded) as they are prohibitively expensive to maintain and extremely dangerous. After getting comfortable with the forced landing and the glide performance the next hurdle is the turn back following an engine failure after take off. The turn back is a wingover with the express purpose of positioning for a landing back on the reciprocal runway from which you departed and is not for the faint hearted. It requires a deft touch and good feel for the aircraft as the manoeuvre is conducted at the maximum lift of the wing called the buffet. This is a complex manoeuvre requiring a lot of forethought of all the considerations and also a lot of practice. You will also be required to commit a number of emergency procedures to memory so that you can act quickly in an emergency, particularly inflight engine relight/restart procedures. In a nutshell it is not easy transitioning to jet aircraft. There are a lot of different considerations that need to be mastered if you are going to become a safe and efficient operator of these marvelous aircraft. Good luck.