diff --git a/src/blackmisc/aviation/selcal.cpp b/src/blackmisc/aviation/selcal.cpp
index 0bac99032..7432e197a 100644
--- a/src/blackmisc/aviation/selcal.cpp
+++ b/src/blackmisc/aviation/selcal.cpp
@@ -21,15 +21,21 @@ namespace BlackMisc
 {
     namespace Aviation
     {
-
-        QList<CFrequency> CSelcal::frequencyEquivalents = QList<CFrequency>();
-        QStringList CSelcal::allCodePairs = QStringList();
-
         QString CSelcal::convertToQString(bool /** i18n **/) const
         {
             return m_code;
         }
 
+        QString CSelcal::unifyCode(const QString &selcalCandidate)
+        {
+            QString s;
+            for (QChar c : selcalCandidate)
+            {
+                if (CSelcal::isValidCharacter(c)) { s += c;}
+            }
+            return s.length() == 4 ? s : QString("");
+        }
+
         bool CSelcal::equalsString(const QString &code) const
         {
             if (code.isEmpty()) return false;
@@ -75,37 +81,39 @@ namespace BlackMisc
 
         const PhysicalQuantities::CFrequency &CSelcal::audioFrequencyEquivalent(QChar c)
         {
-            if (CSelcal::frequencyEquivalents.isEmpty())
-            {
-                QList<CFrequency> frequencies;
-                frequencies
-                        << CFrequency(312.7, CFrequencyUnit::Hz())
-                        << CFrequency(346.7, CFrequencyUnit::Hz())
-                        << CFrequency(384.6, CFrequencyUnit::Hz())
-                        << CFrequency(426.6, CFrequencyUnit::Hz())
-                        << CFrequency(473.2, CFrequencyUnit::Hz())
-                        << CFrequency(524.8, CFrequencyUnit::Hz())
-                        << CFrequency(582.1, CFrequencyUnit::Hz())
-                        << CFrequency(645.7, CFrequencyUnit::Hz())
-                        << CFrequency(716.1, CFrequencyUnit::Hz())
-                        << CFrequency(794.3, CFrequencyUnit::Hz())
-                        << CFrequency(881.0, CFrequencyUnit::Hz())
-                        << CFrequency(977.2, CFrequencyUnit::Hz())
-                        << CFrequency(1083.9, CFrequencyUnit::Hz())
-                        << CFrequency(1202.3, CFrequencyUnit::Hz())
-                        << CFrequency(1333.5, CFrequencyUnit::Hz())
-                        << CFrequency(1479.1, CFrequencyUnit::Hz());
-                CSelcal::frequencyEquivalents = frequencies;
-            }
             int pos = CSelcal::validCharacters().indexOf(c);
             Q_ASSERT(pos >= 0);
             Q_ASSERT(CSelcal::frequencyEquivalents.size() > pos);
-            return CSelcal::frequencyEquivalents.at(pos);
+            return CSelcal::frequencyEquivalents[pos];
         }
 
-        const QStringList &CSelcal::codePairs()
+        const QList<CFrequency> &CSelcal::audioFrequencyEquivalents()
         {
-            if (CSelcal::allCodePairs.isEmpty())
+            static const QList<CFrequency> frequencies(
+            {
+                CFrequency(312.7, CFrequencyUnit::Hz()),
+                CFrequency(346.7, CFrequencyUnit::Hz()),
+                CFrequency(384.6, CFrequencyUnit::Hz()),
+                CFrequency(426.6, CFrequencyUnit::Hz()),
+                CFrequency(473.2, CFrequencyUnit::Hz()),
+                CFrequency(524.8, CFrequencyUnit::Hz()),
+                CFrequency(582.1, CFrequencyUnit::Hz()),
+                CFrequency(645.7, CFrequencyUnit::Hz()),
+                CFrequency(716.1, CFrequencyUnit::Hz()),
+                CFrequency(794.3, CFrequencyUnit::Hz()),
+                CFrequency(881.0, CFrequencyUnit::Hz()),
+                CFrequency(977.2, CFrequencyUnit::Hz()),
+                CFrequency(1083.9, CFrequencyUnit::Hz()),
+                CFrequency(1202.3, CFrequencyUnit::Hz()),
+                CFrequency(1333.5, CFrequencyUnit::Hz()),
+                CFrequency(1479.1, CFrequencyUnit::Hz())
+            });
+            return frequencies;
+        }
+
+        namespace Private
+        {
+            QStringList selcalCodePairs()
             {
                 QStringList pairs;
                 for (int p1 = 0; p1 < (CSelcal::validCharacters().length() - 1); p1++)
@@ -117,10 +125,14 @@ namespace BlackMisc
                         pairs.append(pair);
                     }
                 }
-                CSelcal::allCodePairs = pairs;
+                return pairs;
             }
-            return CSelcal::allCodePairs;
         }
 
+        const QStringList &CSelcal::codePairs()
+        {
+            static const QStringList allCodePairs = Private::selcalCodePairs();
+            return allCodePairs;
+        }
     } // namespace
 } // namespace
diff --git a/src/blackmisc/aviation/selcal.h b/src/blackmisc/aviation/selcal.h
index a53d2480a..cf40298bb 100644
--- a/src/blackmisc/aviation/selcal.h
+++ b/src/blackmisc/aviation/selcal.h
@@ -40,13 +40,10 @@ namespace BlackMisc
             CSelcal() = default;
 
             //! Constructor.
-            CSelcal(const QString &code) : m_code(code.trimmed().toUpper()) {}
+            CSelcal(const QString &code) : m_code(unifyCode(code)) {}
 
-            /*!
-             * Constructor.
-             * Needed to disambiguate implicit conversion from string literal.
-             */
-            CSelcal(const char *code) : m_code(QString(code).trimmed().toUpper()) {}
+            //! Constructor needed to disambiguate implicit conversion from string literal.
+            CSelcal(const char *code) : m_code(unifyCode(code)) {}
 
             //! Is valid?
             bool isValid() const { return isValidCode(m_code); }
@@ -76,14 +73,17 @@ namespace BlackMisc
             //! Audio frequency for character
             static const PhysicalQuantities::CFrequency &audioFrequencyEquivalent(QChar c);
 
+            //! All audio frequencies
+            static const QList<PhysicalQuantities::CFrequency> &audioFrequencyEquivalents();
+
             //! All valid code pairs: AB, AC, AD ...
             static const QStringList &codePairs();
 
+            //! Unify SELCAL code by removing illegal characters
+            static QString unifyCode(const QString &selcalCandidate);
 
         private:
             QString m_code;
-            static QList<BlackMisc::PhysicalQuantities::CFrequency> frequencyEquivalents;
-            static QStringList allCodePairs;
 
             BLACK_METACLASS(
                 CSelcal,