Excel UDF #VALUE! Error - excel

Question

The below Code Counts the number of unique names is a specific column after inputting the data into an array. It works perfectly when running in the the immediate window. But when using as a UDF it throws #Value Error. I am taking all the data into an array and checking the array and getting a number out of it and returning it. I am not modifying any excel sheets or changing the worksheet's environment. Please help!!1
Public Function Operator_Count(Aircraft As String) As Integer
Dim Aircraft_Name As String
Dim Data_Array() As Variant
Dim Row_Count As Integer
Dim Col_Count As Integer
Dim Col_Alph As String
Dim Row_Counter As Integer
Dim Master_Series_Column As Integer
Dim Status_Column As Integer
Dim Operator_Column As Integer
Dim InnerLoop_Counter As Integer
Dim Operator_Array() As Variant
Dim Operator_Array_Transpose() As Variant
Dim Array_Counter As Integer
Aircraft_Name = Aircraft
Operator_Count = 0
'ThisWorkbook.Sheets("Aircraft Data").Activate
Row_Count = ThisWorkbook.Sheets("Aircraft Data").Range("A2", Range("A2").End(xlDown)).Rows.Count
Col_Count = ThisWorkbook.Sheets("Aircraft Data").Cells(1, Columns.Count).End(xlToLeft).Column
Col_Alph = ColumnLetter(Col_Count)
Data_Array = ThisWorkbook.Sheets("Aircraft Data").Range("A1:" & Col_Alph & Row_Count + 1).Value2
For Row_Counter = 1 To Col_Count
If Data_Array(1, Row_Counter) = "Master Series" Then
Master_Series_Column = Row_Counter
End If
Next
For Row_Counter = 1 To Col_Count
If Data_Array(1, Row_Counter) = "Status" Then
Status_Column = Row_Counter
End If
Next
For Row_Counter = 1 To Col_Count
If Data_Array(1, Row_Counter) = "Operator" Then
Operator_Column = Row_Counter
End If
Next
'Resizing the data array
ReDim Operator_Array(0, 0)
'Adding column to the data array
InnerLoop_Counter = 0
For Row_Counter = 1 To UBound(Data_Array)
If Data_Array(Row_Counter, Master_Series_Column) = Aircraft_Name And (Data_Array(Row_Counter, Status_Column) = "In Service" Or Data_Array(Row_Counter, Status_Column) = "On order") Then
Flag = 0
For Array_Counter = 0 To UBound(Operator_Array, 2)
If Operator_Array(0, Array_Counter) = Data_Array(Row_Counter, Operator_Column) Then
Flag = 1
Array_Counter = UBound(Operator_Array, 2)
End If
Next
If Flag <> 1 Then
ReDim Preserve Operator_Array(0, InnerLoop_Counter)
Operator_Array(0, InnerLoop_Counter) = Data_Array(Row_Counter, Operator_Column)
InnerLoop_Counter = InnerLoop_Counter + 1
End If
End If
Next
Operator_Count = UBound(Operator_Array, 2)
End Function
Function ColumnLetter(ColumnNumber As Integer) As String
Dim n As Integer
Dim c As Byte
Dim s As String
n = ColumnNumber
Do
c = ((n - 1) Mod 26)
s = Chr(c + 65) & s
n = (n - c) \ 26
Loop While n > 0
ColumnLetter = s
End Function