In the common law of bailments, the term "lien" is any amount owed by the bailor to the bailee for services provided while the property is in the bailee's possession. I didn't mean it in this instance to be the result of a filing a legal action pursuant to statutory power of sale.
It appears the shop was not in possession of the aircraft at the time. It's one thing to keep it locked up in your hangar. It's quite another to try to wrest possession away from the rightful owner. You just can't go repo a plane on a lien without perfecting it.
Neither can an bailor take his property from the bailee without paying what is owed, that is clear in the PA statute I linked to. The bailee has the right to refuse to deliver or surrender it voluntarily, which was clearly his choice in this case . No different than trying to take your car from a parking garage without paying the ticket. You can't just blast through the barrier, even though it's your car. The garage must raise the gate and allow passage.
If I was the shop I'd try and get what I could while the plane was here. Then if I thought I was in the right I contact a lawyer about theft of services, small claims, lien etc
Unless someone stole the aircraft and delivered it to the shop for improved avionics without the owner's knowledge, there was a contract for bailment, written or not. Not really difficult to prove. If it were me, I would have made clear by text or email the aircraft wasn't to leave my custody until the bill was paid or a bond posted for the disputed amount in the local court of jurisdiction. If the the owner refused the bond, I would keep possession and follow closely the state statutory procedure for notice pursuant to a lien with statutory power of sale, and add a claim for compensatory damages for storage and any other charge allowed by law.
If I were the owner, I would gladly post the bond, because if the shop were committing a fraud, that will become clear in court at the bond hearing as well and could cost the shop big in punitive and compensatory damages and legal fees.The bond hearing would guarantee both sides would show at trial, or settle, because the shop owner isn't getting paid until they prove their claim is valid in court, and if the owner feel he's being defrauded and can prove it, the shop will eat the costs anyway. And in the meantime he can enjoy his new panel etc.
My business is by it's nature bailment. I have been down this road too many times with owners trying to get out of paying a bill. If you let your security interest go down the road, by the time you hire the lawyers to pursue the claim in court, you're spending $5k to collect $10k. Bailment is one of the oldest areas of the law, and well travelled ground.