How to create right relations between models in Django project

I am developing Django progect with following models:

class Aircraft_fleet(models.Model):
    Aircraft_type = models.CharField(max_length=16)
    Aircraft_Serial_Number = models.CharField(max_length=16)
    Engine_1_Ser_Num = models.ForeignKey('Engine', related_name='Engine_1', on_delete=models.PROTECT)
    Engine_2_Ser_Num = models.ForeignKey('Engine', related_name='Engine_2', on_delete=models.PROTECT)

class Engine(models.Model):
    Ser_Num = models.PositiveIntegerField()
    Flight_Num = models.CharField(max_length=80)
    Flight_Date = models.DateField()
    Oil_Quantity_Departure = models.DecimalField(max_digits=5, decimal_places=2)
    Oil_Quantity_Arrival = models.DecimalField(max_digits=5, decimal_places=2)
    Oil_Temperature_Departure = models.DecimalField(max_digits=5, decimal_places=2)
    Oil_Temperature_Arrival = models.DecimalField(max_digits=5, decimal_places=2)

One Aircraft (Aircraft_fleet model) have two Engine Ser_Num (different of course). Particular Engine Ser_Num (Engine model) is fitted only on one partucular Aircraft.
The thing is that in database, "Engine" table has multiple records for each Engine "Ser_Num".
See picture below:
Engine Model in database look like:

I can't determine how to create the proper relations between these two models.

Currently, relation is one to many, but it doesn't work. Following error apears:

django.db.utils.IntegrityError: The row in table 'oilcons_aircraft_fleet' with primary key '1' has an invalid foreign key: oilcons_aircraft_fleet.Engine
_2_Ser_Num_id contains a value '193262' that does not have a corresponding value in oilcons_engine.id.

Okay if I understand you correctly the engine_ser_num does not belong to the Aircraft_fleet but to the Engine.

Go with this:

class Aircraft_fleet(models.Model):
    Aircraft_type = models.CharField(max_length=16)
    Aircraft_Serial_Number = models.CharField(max_length=16)

class Engine(models.Model):
    aircraft = models.ForeignKey('Aircraft_fleet', on_delete=models.CASCADE)
    Ser_Num = models.PositiveIntegerField()
    Flight_Num = models.CharField(max_length=80)
    Flight_Date = models.DateField()
    Oil_Quantity_Departure = models.DecimalField(max_digits=5, decimal_places=2)
    Oil_Quantity_Arrival = models.DecimalField(max_digits=5, decimal_places=2)
    Oil_Temperature_Departure = models.DecimalField(max_digits=5, decimal_places=2)
    Oil_Temperature_Arrival = models.DecimalField(max_digits=5, decimal_places=2)

Now one Engine can be related to one Aircraft_fleet

e = Engine.objects.all().first()
print(e.aircraft.Aircraft_type)

And one Aircraft_fleet can be related to multiple Engines

af = Aircraft_fleet.objects.all().first()
for engine in af.engine_set.all():
    print(engine.Ser_num)

Let me know how it goes.

important edit:

For sure you need to do the migrations correctly and migrate it. Because that IntegrityError could also be caused by "old" data inside the database that still have "old" relationships.

possible structure:

class Aircraft(models.Model):
    Aircraft_type = models.CharField(max_length=16)
    Aircraft_Serial_Number = models.CharField(max_length=16)

aircraft may have 2/3/4 ... engines:

class Engine(models.Model):
    Ser_Num = models.PositiveIntegerField()
    aircraft = models.ForeignKey('Aircraft', ....)   # engine belongs to an aircraft
    ...

class Flight(models.Model):
    Flight_Num = models.CharField(max_length=80)
    Flight_Date = models.DateField()

class EngineDataRecord(models.Model):
    flight = models.ForeignKey('Flight', ....)     # Engine Data record is for a certain flight
    engine = models.ForeignKey('Engine', ....)     # and a certain engine
    Oil_Quantity_Departure = models.DecimalField(max_digits=5, decimal_places=2)
    Oil_Quantity_Arrival = models.DecimalField(max_digits=5, decimal_places=2)
    Oil_Temperature_Departure = models.DecimalField(max_digits=5, decimal_places=2)
    Oil_Temperature_Arrival = models.DecimalField(max_digits=5, decimal_places=2)