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Warranty Reserves Estimated Cost (Details) - USD ($)
12 Months Ended
Apr. 30, 2020
Apr. 30, 2019
Warranty Reserves    
Changes in estimates related to pre-existing warranties $ (189,000) $ 491,000
Product Warranty Expense 288,000 240,000
UAS    
Warranty Reserves    
Product Warranty Expense 0 251,000
UAS | Warranty reserve.    
Warranty Reserves    
Changes in estimates related to pre-existing warranties   $ 491,000
Revised warranty reserve charges $ 302,000