What are the advantages/disadvantages of ultrasonic in aircraft structure inspection?
Ultrasonic inspection is perhaps the most widely used inspection procedures in aerospace to detect subsurface anomalies. 


Portability- A number of portable ultrasonic equipment are available, which are very simple and are widely used.
The detection depth for flaws is far superior compared to most other NDT methods.
It can be used for detecting flaws even if only one surface of the part is accessible (simply use pulse echo method).
The results are more or less instantaneous- Usually there is no waiting time involved as the results are displayed in a screen.
The time required for preparation is very less; usually the part is cleaned and the couplant applied; after the inspection, it can simply be wiped off. 


However, just like any other (NDT) method, this too has its own share of disadvantages:


It is very difficult to use this method for parts which are very thin- for e.g. the aircraft skin because of the dead zone, though this can be overcome by using higher frequencies or immersion technique; in most cases of aircraft checks, this may not be an option. Also, it cannot be used for irregular and non-homogeneous parts (At-least, not the portable methods).
The method does not detect flaws that are oriented parallel to the wave propagation.  
The material must be characterized prior to inspection. As the returning  signal is affected by attenuation within the material, comparisons can be made only if a similar (defective and good) specimen of same material is characterized first.
Usually, Ultrasonic method requires a trained operator for interpretation of the results.


As far a full aircraft is considered, only some critical parts are tested using ultrasonic (or other NDT) methods. There is no way to check the entire aircraft except by thorough visual inspection.