Consider a fighter jet flying formation with an airliner-size aircraft, such as during an interception or mid-air refueling.
Are there areas which should be avoided by the fighter jet? I.e. are there certain positions where the downdraft or turbulence caused by the larger aircraft would send the small aircraft out of control?
In higher altitude the wake turbulence moves downward behind the aircraft. So as the pilot of the lighter aircraft behind the bigger you should keep the same level at least.
photo by: Steve Morris, image source
I suppose the wake turbulence only becomes dangerous at some distance, when the downwash forms vortices. – Sanchises
No. The wake turbulence starts forming at about 2/3 of the wing span (measured from body). It starts as a real narrow but powerful vortex. Then it raises its diameter losing power (slowly).
photo by: Josef Willems, image source
As you can see near the aircraft the wake turbulence has a sharp profile and is quite predictable. Therefore there is a safe area directly behind the tanker.
related post: How does an aircraft form wake turbulence?
In case of air to air refueling, the 'receiver' (the aircraft being refueled) should be careful about not getting into the wake of the tanker, which is usually much larger and heavy. For example, the NATO air-to-air refueling manual, ATP-56 specifically states:
The area of extreme turbulence directly behind and slightly to the right of the tanker should be avoided. Blade stall and uncontrolled settling may be encountered if this area is entered.
The receiving plane is usually flown below the wake turbulence of the tanker, in both the hose & drogue and flying boom methods. Usually, the receiver is directly behind and around 10' below the tanker. For example, see How do aircraft refueling in-air deal with wake turbulence?
In addition to turbulence, in case of close formation flying, there are other things to contend with, as explained in Advances in air to air refuelling:
In addition to the trailing air vortices the receiving aircraft must contend with downwash of airflow from the tanker. The affected local airflow typically exhibits a nose-down pitching moment on the receiver which, in turn, affects its speed and approach rate.
Not only the receiver, the tanker should also be careful about flow disturbances.
... mostly between large transport aircraft, the lead aircraft experiences a change in its airflow caused by the bow wave, or upwash, from the receiver. This increases the angle of attack around the tanker's tail which also results in a pitch-down motion.
Under-running the tanker is particularly dangerous since the downwash effect will dissipate, resulting in the receiver pitching up. At the same time a tanker on altitude hold may instigate a nose-down pitch due to a perceived climb due to the decrease in pressure in the surrounding air, potentially leading to collision.
The figure below shows some regions where disturbances would be encountered aft of a tanker.
Typical spanwise airflow disturbances from a trailing tanker wake.; image from Advances in air to air refuelling by Peter. R. Thomas et. al.
In addition, the trailing aircraft should also be careful about the jet exhaust.
Downwash area to the left and the Backwash area to the right, as though that had been rotated 90° CCW. It wasn't until I started typing this that I realized that Downwash was, in fact, to the 'left', closer to the plane & vortex lines, and that Backwash was pointing away from the direction of flight. I knew that's how it should be, my eyes just refused to see it that way... Time for more coffee.
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A classic example was the tragic accident in 1992 of a T-34 trainer getting too close to an F-14 while attempting to visually inspect a landing gear that had "unsafe" indications in the cockpit. The T-34 is quite a bit smaller than an F-14. Each aircraft will have a turbulent flow signature similar to the illustration aeroalias provided for the tanker in his answer (influenced obviously by airfoil shape and gross weight of the aircraft). If you approach an aircraft that is not the same as yours, and are not familiar with it's wake areas, it is possible to make a fatal mistake.
14 Jan. 1992: ... a T-34C over Corpus Christi, TX. The Tomcat had an unsafe landing gear and the T-34 was checking it and later wrapped itself around the Tomcat's right wing. Both aviators in the T-34 were killed. The wreckage was left on the Tomcat's wing. The Tomcat land at the NAS there but had extensive damage.
Back when I was flying T-34's (some years before that accident) I did any number of gear checks on other T-34's and a couple of times T-44's at that same naval air station. (The unsafe gear pattern was 2000' AGL). What most likely caught the T-34 pilot in this case was using a similar approach (in terms of geometry) when joining up on an aircraft of considerably greater weight and a different turbulent signature, and then not having enough altitude to recover from the loss of control when the upset happened.
As the photographs illustrate, the disturbed air zones should inform how one makes an approach when closing with another aircraft. There's a good reason aerial refueling is done at a high altitude.
