The first class medical is not nearly as bad as some make it out to be. Over the years I have met more than one that knew how to work the system and got around many issues they had. Obviously they are counting on the applicant to honestly and truthfully come forward if any problems, issues, conditions etc, are present. Many of the "disqualifying" factors are not very evident to the ame at the time of the exam, that is one of the reasons why you have to fill out the application that includes many yes/no type answers. As for the drug test, I do believe that they test the urine for certain types of drugs that could be present in the body within a certain time frame, some are not detectable by this test and would not be found by a simple urine test. They will do the hernia thing as well as feeling around the stomach area for anything that would be obvious to the doctor. Once again they are counting on you to come forward should you have any problems that were noticed by you. They will test the flexibility with an instrument that they use to hit (stimulate) certain areas of the arms, legs, etc. Depending on the severity and extent of your back problems it is somewhat hard to say but I know many older pilots that routinely suffer these types of problems and they have had no problems being able to hold and maintain a first class medical within the prescribed tolerances/standards. You may (or may not) want to discuss this issue with the ame, I personally would seek the advice/input from a doctor who specializes in the field versus asking the ame. Sometimes it is better to not open a possible can of worms if you can avoid doing so. Many of the ame's are just general family doctors and will not be able to offer much advice other than to possibly suggest another doctor(s) who practices this type of medicine. They will do the vision test and if you look in the far's you will see what is needed to be granted a first class medical. The blood test supposebly is something that is in the works for general "issuance" of the med- I have heard that certain people with certain pre-existing conditions must submit to a blood test upon request from someone within the FAA. The older you get the more detailed this exam will probably become. The EKG, etc, ..
I did get a laugh from a buddy who is at AE when he told me how they conduct the company physical, talk about leaving nothing out...
good luck
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